1. Introduction
Aluminum ranks as the second most widely used metal globally, valued for its exceptional specific strength, stiffness, thermal and electrical conductivity, formability, and corrosion resistance [1,2,3,4]. Following steel, it stands as the most produced metal and the leading non-ferrous metal by volume [5,6], surpassing all others combined [7]. In aerospace and automotive sectors, aluminum increasingly substitutes steel, reducing greenhouse gas emissions significantly compared to traditional materials [8,9]. Its extensive use in construction, packaging, aerospace, automotive, and electrical sectors, coupled with its recyclability, underscores its economic and environmental importance [10]. Remarkably, 75% of all aluminum ever produced remains in circulation today [11,12].
A model in aluminum industry highlights the benefits of increasing the use of recycled material, reducing production costs and carbon dioxide emissions [13]. Demand for aluminum metal matrix composites (AMCs) continues to grow, with powder metallurgy proving effective in enhancing properties by eliminating porosity and casting defects [14]. These composites, reinforced with hard ceramics, enhance strength, hardness, wear resistance, and thermal stability [15,16,17].
Aluminum alloys, known for their low density, corrosion resistance, and mechanical properties, are favored as matrix materials in composites for applications in automotive, military, aerospace, and electronics industries [10,18,19,20,21]. Various fabrication methods, such as stir casting [22,23,24] and powder metallurgy, address different challenges [14,25,26,27]. Powder metallurgy (PM), for instance, offers lower processing temperatures and more cost-effective production [28].
Currently, additive manufacturing (AM) processes are an emergent technique. Metal powders used in powder bed fusion AM machines must have specific physical and chemical characteristics to ensure consistent and reliable printing results. Key attributes, similar to those required in PM, include particle morphology, particle size distribution, density, porosity, flowability, satellite formation, agglomeration, humidity sensitivity, filler properties, and chemical composition [28]. These characteristics must be thoroughly tested and optimized for each AM process or technology to produce high-quality, dependable parts. Research in metal AM predominantly focuses on titanium, nickel, and aluminum alloys, which find applications in aerospace, automotive, and other industries. The adoption of additive technologies is motivated by the flexibility in design and the growing demand for lightweight materials that offer enhanced final part functionality [29,30].
However, aluminum and its alloys exhibit poor wear resistance, limiting their use in high-performance tribological applications [31,32,33]. Ceramic-reinforced AMCs such as SiC, Al2O3, and B4C provide improved wear resistance over monolithic materials [15,16,30,31,33,34]. Incorporating materials with high lubricating properties, such as graphite, enhances the self-lubricating capabilities of AMCs, reducing friction and improving component lifespan [35,36]. Studies show that hybrid metal matrix composites reinforced with graphite and Al2O3 particles significantly enhance wear and mechanical properties [37].
After reviewing the techniques for obtaining hybrid materials composed of aluminum and ceramic, powder metallurgy was chosen as the method to create composites of aluminum with inorganic natural pigments.
Some natural vegetal pigments, like betalains, anthocyanins, chlorophylls, and ca-rotenoids, can form complexes with metals, enabling their use to detect heavy metals in the environment under aqueous conditions and typically at room temperature [38]. Biosensors have been developed to measure color changes caused by metal interactions, with color sheets useful for in situ detection [39]. One such pigment, flavylium from pomegranate, enhances light harvesting in dye-sensitized photovoltaic cells due to its strong chelation with TiO2, improving energy conversion [40]. Natural pigments from plants are efficient sensitizers for solar cells because of their high light absorption, low extraction cost, and low toxicity [41]. These pigments are also used as food colorants, but have low stability compared to synthetic colorants, which can be improved with copigments like polymers and metals [42].
In the field of health applications, the production of pigments using microorganisms offers an economical alternative to synthetic options, with environmental and pharmacological benefits, such as combating antibiotic resistance. However, challenges like low yield and safety need to be addressed for industrial use [43].
Inorganic mineral pigments are used in art, often mixed with natural resins and drying oils. Studies have examined the durability of the metallic soaps formed from reactions between pigment cations and the oil’s free fatty acids. Zinc white and enamel act as catalysts, promoting the formation of oxalates, which impact varnish removal and cleaning processes. These oxalates permanently alter the interface between paint and varnish layers, as well as the aesthetic appearance of painted surfaces [44].
These natural pigments are ceramic in nature, such as carbonates, sulfates, oxides, or sulfides [45,46], and, to the best of our knowledge, have not yet been used as metal additives. While these pigments do not inherently enhance mechanical properties, they may improve hardness or wear resistance if they form intermetallic compounds with aluminum. Intermetallic compounds perform well in high-temperature environments, with advantages like low density, high melting points, and excellent corrosion resistance. However, they suffer from poor fracture toughness at low temperatures, where cracks form more easily due to inelastic deformation. In pure metals, grain boundaries resist cracking, but in disordered alloys, impurities and alloying elements can lead to embrittlement [47].
Intermetallic compounds can form through solid-phase diffusion, creating small particles, provided the alloying agent remains in the metal matrix. As the temperature rises and the alloying agent diminishes or disappears, these intermetallic particles increase in size [48]. Additionally, the presence of graphite or carbon can promote the formation of eutectics and liquid phases, or influence the kinetics of the dissolution of a metal within the grains of another metal, enhancing the diffusion of the alloying agent and accelerating intermetallic formation [49,50].
Therefore, the main objective of this study is to produce colored aluminum samples via powder metallurgy without compromising their mechanical properties. The novelty of this work lies in the use of natural pigments to impart color. This research may serve as a foundation for additive manufacturing, offering design freedom for producing colored gas burners for kitchens.
Colored aluminum parts have the advantage that the color permeates the entire material, not just the surface, as is the case with anodized color finishes [51]. Anodizing is a surface treatment used to enhance the corrosion resistance of aluminum and its alloys through an electrochemical process. This process involves acids (such as sulfuric or phosphoric acid) and deposits a highly ordered and rough oxide layer on the surface. Before anodizing, a pretreatment removes the natural oxide layer that spontaneously forms on aluminum. The pretreatment involves a chemical etch with sodium hydroxide (5% NaOH at 60 °C for 5 min), followed by neutralization with nitric acid (50% HNO3 for 2 min) and rinsing with distilled water. After anodizing, the parts are immersed in a dye that penetrates the oxide layer, imparting color.
Although there are no wear studies specifically on color-anodized aluminum, some studies compare uncoated aluminum to anodized aluminum [52]. In this study, sulfuric acid anodizing is compared with oxalic acid anodizing. Surface hardness increases when sulfuric acid is used, with optimal parameters being a concentration of 180 g/dm3, a current density of 35 mA/cm2, and a bath time of 50 min. For oxalic acid, surface hardness improves with a concentration of 30 g/dm3, a current density of 10 mA/cm2, and a bath time of 70 min. All coatings show better wear resistance compared to the 6061-T6 aluminum substrate.
Notably, the study conducted by J. Nam and J. Kang [53] found that uncoated aluminum exhibited rapid wear from the start of the test. In contrast, the anodized aluminum coating protected the friction surface at the beginning of the test, delaying surface damage caused by friction. However, as friction continued, the coating surface experienced damage due to material transfer. As a result, the coefficient of friction gradually increased on the anodized aluminum, eventually causing the coating to lift and expose the underlying aluminum.
Therefore, a key advantage of aluminum colored through powder metallurgy over anodized aluminum coatings is that the color permeates the entire material, rather than being limited to the surface. In surface-only coloring, friction can wear away the coating, exposing the raw aluminum underneath. The primary limitation of this method is the limited availability of natural pigments, which must be tested and validated.
2. Materials and Methods
2.1. Materials
The aluminum used as composite matrix is an irregular, water-atomized powder with a particle size ranging from d10 = 20 µm to d90 = 80 µm, with a d50 = 50 µm, and a purity exceeding 99.5%. This powder was supplied by Sales Manager, AMG Alpoco UK Limited—AMG Advanced Metallurgical Group N.V. (Anglesey, UK).
The pigments used included various metal oxides and sulfides, supplied by Manuel Riesgo, S.A. (Madrid, Spain). These pigments were:
Cadmium sulfides (cadmium red medium, cadmium orange light, and cadmium yellow lemon) (Figure 1);
Metal oxides (Chromium green, Mars red, BayFe red, and Cobalt blue) (Figure 2);
Other: Mineral blue and chypre Limonite ochre.
During preliminary tests, the cadmium sulfide and mineral blue pigments were discarded due to color changes observed after heating at 630 °C for 1 h.
Details of selected pigments:
Limonite ochre is a natural inorganic pigment with a yellowish earth color (Figure 2a). It consists of aluminum silicate (clay), silicon dioxide (quartz), and calcium salts, colored by ferric hydroxide (iron ore). This pigment offers good light resistance, is relatively harmless, and has a density of 0.99 g/cm3. Before mixing with aluminum, the pigment underwent a color modification process involving heating at 350 °C for 1 h. The loss of water produces a light brown color (Figure 2b).
Deep Mars red is a synthetic inorganic pigment made from iron oxide (Fe2O3). Its color varies from bluish to yellowish red (Figure 2c). The pigment is characterized by high opacity, excellent tinting strength, and outstanding lightfastness. It is also relatively non-toxic.
BayFe red is another synthetic iron oxide pigment (Fe2O3) (Figure 2d).
Deep Cobalt blue, also known as Royal blue, is cobalt aluminate (CoAl2O4), an inorganic pigment with moderate tinting strength (Figure 2e). It offers good lightfastness and poses minimal toxicity risks.
Chrome green is chromium sesquioxide (Cr2O3), which appears as a pale yellowish-green color (Figure 2f). It is opaque with weak tinting power, but boasts good lightfastness and is relatively non-toxic.
2.2. Sample Preparation
The composites were produced by milling 10 wt% of pigment with aluminum powder. The mixing was performed using mechanical ball milling for 10 min with 100 mm diameter balls in a 1:1 ball to power ratio in weight. A PM100 planetary ball mill, supplied by Retsch GmbH (Haan, Germany), was used.
The composite powders were then uniaxially compacted at 500 MPa using a 10-ton press (Microtest, Madrid, Spain) with floating dies to form the pieces.
After preparing the green samples, they were sintered in a stainless steel furnace with temperature control ranging from 400 to 1250 °C, also supplied by Microtest (Madrid, Spain). The sintering atmosphere was nitrogen (N2), maintained under overpressure by continuously bubbling the gas through water. The sintering cycle involved heating at 5 °C/min from room temperature (23 °C) to 610 ± 3 °C, holding at this temperature for 30 min, and then cooling at 20 °C/min down to 30 °C.
2.3. Color Study
Since aesthetics are a primary concern, the initial evaluation focused on the resulting color of the sintered materials and how it differs from the base material, which in this case is sintered aluminum.
Color systems integrate information from three components: the light source, the observer, and the object. These systems provide the framework for discussing colors, analyzing color differences, and documenting them. The color system recommended by the CIE (International Commission on Illumination) [54] and widely adopted globally is the CIELab system [55] (Figure 3).
In the CIELab system, each color can be identified by its L*, a*, and b* coordinates. Alternatively, the chroma (C*) represents the saturation or intensity of the color (Equation (1)), while the Hue angle (h) corresponds to the actual chromaticity or hue of the color (Equation (2)). The asterisk (*) in the notation signifies that it refers to a CIELAB system. A ColorEyé XTH device, supplied by Neurtek Instruments (Eibar, Guipúzcoa, Spain), was used to measure these parameters.
(1)
(2)
2.4. Physical Properties
2.4.1. Green Properties
The dimensional changes and density of the produced materials were evaluated following the guidelines of the UNE-EN ISO 4492:2018 [56] standard. Dimensional variation was assessed by comparing the dimensions of the green samples with those of the compacting die (31.40 ± 0.05 mm × 12.60 ± 0.05 mm and 25 ± 0.05 mm × 25 ± 0.05 mm). This elastic deformation, commonly referred to as spring-back, was observed. Additionally, the die walls were lubricated using zinc stearate to facilitate the process. The green density was also calculated based on the weight and dimensions of each sample.
2.4.2. Sintering Properties
For the sintering samples, density and dimensional changes were also measured. The relative density was calculated according to density of the pigments, 4.21 ± 0.06 g/cm3 for Limonite ochre, 4.23 ± 0.04 g/cm3 for Mars red and BayFe red, 3.80 ± 0.05 g/cm3 for Cobalt blue, and 5.20 ± 0.05 g/cm3 for Chrome green. Aluminum metal has a density of 2.70 ± 0.03 g/cm3.
Pigment density was measured using a Micromeritics AccuPyc 1330 helium pycnometer (DataPhysics, Neurtek Instruments, Eibar, Spain). Three samples of each pigment were analyzed, with each measurement repeated five times.
2.5. Mechanical Properties
2.5.1. Bending Behavior
Three-point bending tests were performed in accordance with the UNE-EN ISO 3327:2010 [57] standard using a universal testing machine supplied by Microtest S.A. (Madrid, Spain). The sample dimensions were 31.40 ± 0.05 mm × 12.60 ± 0.05 mm. The test is carried out with a 5 kN load cell and a displacement speed of 0.1 mm/min. The bending strength is obtained from the applied load using Equation (3). The percentage elongation is calculated according to Equation (4).
(3)
(4)
where F is the force applied during the test, L is the gauge length (for the tooling used, this value is 25.0 ± 0.5 mm), w is the width of the test piece, h its thickness, and δ the deflection until failure data.2.5.2. Wear Resistance
Dry wear tests were conducted at room temperature using a pin-on-disk tribometer (Microtest, Madrid, Spain) following the ASTM G99-05:2010 [58] standard. The pin consists of a 6 mm diameter chromium ball.
The friction coefficient (FC) was also determined during the wear test from the FC versus sliding distance curve. The FC value was taken from the constant line observed after the initial peak, which corresponds to the formation of the wear track. Test conditions were 240 rpm, with an applied load of 5 N, and relative humidity below 30%. The sliding distance was 400 m. Wear was assessed by weight loss and calculated density, using Equation (4). For each material, three samples were tested.
2.6. Microstructural Analysis and Thermal Performance
The surfaces of the samples and wear tracks were studied by scanning electronic microscopy (SEM) (Philips X-30, Philips Electronic Instruments, Mahwah, NJ, USA). Additionally, SEM the chemical composition of the sample surfaces was analyzed to identify the presence of different elements using EDX (Energy Dispersive X-ray Spectroscopy), which was performed with the same SEM equipment.
To investigate the potential diffusion of pigments into the aluminum matrix and the formation of new compounds, the composites were analyzed by X-ray diffraction (XRD). The XRD was performed using a Philips PANalytical X’Pert-MRD system (Malvern PANalytical Limited, Malvern, UK), acquiring intensity versus 2θ plots within an angular range of 10° to 90°, with a step size of 0.02°, and a step time of 2.4 s per step. The X-ray wavelength is determined by the available anode materials, in this case, a copper anode with λ = 1.54 Å. The generator voltage was set to 40 kV.
The characterization of the diffractogram peaks was carried out using reference codes from the equipment’s library, identifying aluminum, AlCr and AlCo intermetallics, as well as specific oxides, based on the identified peaks.
Given the potential applications of these materials, thermal cycle tests were performed on the best-performing materials. These tests were conducted in an oven at 300 °C without a controlled atmosphere. Three samples of each selected material were placed in the preheated oven for 30 min, then removed and allowed to cool for 30 min before being reintroduced for another cycle. This process continued up to 160 cycles.
The samples were weighed, measured, and their color was recorded at the start of the test and after 25, 50, 100, and 160 cycles. The thermal cycles were selected based on practical considerations, as 25 cycles correspond to approximately 50 h, or a little over two days of testing.
The oven used was a Lenton, equipped with Eurotherm temperature control, and supplied by Biometa Tecnología y Sistemas, S.A. (Llanera, Spain).
3. Results
3.1. Pigment Selection and Color Study
The oxide pigments (Figure 4a) were subjected to an oxidation process to assess any color changes. As depicted in Figure 4b, mineral blue and chypre Limonite ochre undergo color changes, while natural azurite is completely oxidized. However, when azurite is mixed with aluminum alloy, it retains its color. This is likely due to aluminum acting as a reducing agent, preserving azurite’s structure, and thereby maintaining its color. Nonetheless, the hue is not very intense due to its hydrated copper carbonate nature, being replaced by the more vivid and stable Cobalt blue pigment. Conversely, the sulfide pigments completely oxidized and decomposed upon heating. Consequently, other red oxides such as Mars red and BayFe red were introduced (Figure 2c,d).
The MMCs were analyzed to identify any color changes after uniaxial compaction and sintering. Figure 5 displays the MMC samples with various pigments. The brightness and contrast in Figure 5 make it challenging to perceive the colors accurately. However, the red hues appear quite dark due to their intensity (Figure 5c,d). In contrast, the Al–Limonite ochre sample (Figure 5b) shows the least difference compared to the aluminum sample (Figure 5a). The palest hues are observed in the Al–Cobalt blue and Al–Chrome green samples (Figure 5e and Figure 5f, respectively).
Table 1 lists the parameters of the CIELab system, including the calculated color saturation (intensity) and chromaticity. Typically, when a standard is used for comparison, in this case sintered aluminum, the differences in color parameters are highlighted. Variations in the ΔE* parameter (Table 2) greater than 10 are visible to the naked eye, and values exceeding 20 indicate a noticeable color change. It can be observed that Al–Mars red and Al–BayFe red exhibit the most significant color variations due to saturation (C*), while Al–Chrome green shows the greatest difference in hue (h). As a result, the colors in the various manufactured composites are clearly distinguishable.
3.2. Physical Properties
The physical properties evaluated included the density of the green compacts (pre-sintered samples) and post-sintering, along with the dimensional variation. Regarding density (Figure 6a), there are generally no significant differences between the sintered materials and the green (unsintered) compacts. This suggests that no swelling occurs due to particle relaxation, nor is there significant density loss from oxidation during sintering. The only exceptions are pure aluminum, which decreases in density by 1.35%, and the composite with Limonite ochre, which shows a 0.9% reduction. For the other composites, the density increases by 0.7%, 1.9%, 0.1%, and 0.8% for Al–Mars red, Al–BayFe red, Al–Cobalt blue, and Al–Chrome green, respectively. The addition of oxides does not significantly alter the density of aluminum; the composites with Limonite ochre and Cobalt blue show the greatest decreases of 7.4% and 4.0%, while Mars red and BayFe red show reductions of less than 1%. Notably, the Al–Chrome green composite shows a 2% increase in density. This suggests that the low-density advantage of aluminum is not significantly affected by the addition of these pigments.
Regarding relative density (Figure 6b), a decrease of 1% is observed in aluminum (from 99% to 98%) and in Al–Limonite ochre (from 88% to 87%) during sintering. Al–Mars red remains constant at 93%, while Al–BayFe increases by 2% (from 92% to 94%). Al–Cobalt blue increases by 1% (from 92% to 93%), and Al–Chrome green also increases by 1% (from 94% to 95%). These relative density values provide information about the porosity of the materials, which is 2% in aluminum and ranges between 5% and 8% in aluminum composites. The other measured physical property was dimensional variation, a critical factor in part design. This property was studied in terms of both thickness and width, as the parts are square. As shown in Figure 6c, width exhibits minimal dimensional changes, with extremes of 0.20% for the Al–BayFe red composite and −0.03% for the Al–Limonite ochre composite. For thickness, pure aluminum shows a change of 1%, while the Al–BayFe red composite contracts by 2.25%. The other composites exhibit shrinkage below 1%, with Al–Cobalt blue having the lowest at 0.37%. Typically, variations of less than 1% are required for most applications. Therefore, all composites except Al–BayFe red are expected to perform adequately in practical applications.
3.3. Bending Properties
Figure 7a depicts the bending strength of aluminum and composites. Al–Limonite ochre, Al–BayFe, and Al–Chrome green exhibited lower bending strength compared to pure Al, by 44%, 82%, and 30% respectively. Al–Mars red and Al–Cobalt blue showed slight increases in bending strength of 1% and 5%, respectively; however, due to high measurement error, these composites can be considered statistically similar to pure Al.
As with all metal–ceramic composites, the material’s brittleness increases, resulting in lower material deformation, as observed in Figure 7b. Consequently, the elongation decreased for all composites by around 90%, except for Al–Chrome green, which decreased by 72%. This reduced elongation is indicative of increased brittleness. As a result, the bending modulus increased approximately 160% for Al–Limonite ochre, Al–Mars red, and Al–Cobalt blue, while Al–BayFe and Al–Chrome green increased by 41% and 77%, respectively. This increased modulus reflects the higher brittleness compared to pure aluminum, with bending modulus values ranging from 0.65 GPa in pure Al to 1.70 GPa in the stiffer composites (Figure 7c).
The microstructural characteristics of these materials offer further insights into their mechanical properties, which will be analyzed in detail in Section 4
3.4. Wear Behavior
All composites exhibited greater wear compared to pure Al (Figure 8a), with the exception of Al–Chrome green, which demonstrated wear resistance similar to that of the aluminum. The most significant increases in wear were observed in Al–Limonite ochre and Al–BayFe red, with wear rates increasing by approximately 1000%; Al–Mars red showed a 500% increase in wear, while Al–Cobalt blue exhibited a more moderate increase of 100%.
Despite these variations in wear, similar FCs (Figure 8b) were measured for aluminum and its AlMCs, all around 0.7—a typical value for powder metallurgical materials. Stronger interparticle bonds in the composites could potentially reduce the FC, as some pigments may act as lubricants in the wear process due to their unctuous properties.
The wear tracks of pure aluminum (Figure 9a), along with the composites containing Cobalt blue (Figure 9e) and Chrome green (Figure 9f), exhibited similar track widths, ranging between 2 and 2.3 mm, indicating comparable wear resistance. However, the composites with Limonite ochre (Figure 9b), Mars red (Figure 9c), and BayFe red (Figure 9d) displayed wider tracks, between 3 and 3.5 mm, demonstrating lower wear resistance.
Two types of wear were also observed in the tracks (Figure 9): abrasive (indicated by blue squares) and adhesive (indicated by red squares). Abrasive wear predominated in the composites with the highest wear, as shown in Figure 8a. The different types of wear will be discussed in more detail in Section 4.
4. Discussion
The CIELAB system, particularly using the ΔE* parameter, provides essential information to assess whether there is a chromatic difference between a standard and a studied sample. When ΔE* exceeds 20, a significant color change is observed, becoming perceptible to the human eye. Based on this parameter, the only pigment excluded as a viable colorant is Limonite ochre, as Cobalt blue and Chrome green have ΔE* values near 20, while the red pigments approach values of 30.
As shown in Figure 6b, the relative density of pigmented aluminum is lower than that of pure aluminum, resulting in a porosity of 5–8%, compared to only 2% for pure aluminum. This porosity is evident in micrographs. In some cases, the porosity is small and rounded, which is typical of materials produced by powder metallurgy. However, a lack of cohesion between aluminum grains is also observed, likely due to poor adhesion with the pigment.
This porosity could potentially be reduced by using double punches during compaction or through hot compaction. Processes like gas isostatic forging (GIF) or hot isostatic pressing (HIP) can also help close the pores. Generally, during the initial stage of sintering, porosities transition from interconnected (open) to isolated (closed) as densification progresses. It is well-established that in sintered products, this transition occurs at around 6% porosity. Whether the transition happens within the 6–8% range depends on the processing conditions and material properties [59,60].
In the materials studied, this porosity closure does not occur. Increasing the sintering temperature for pigmented aluminum could potentially reduce porosity and result in closed pores. Similarly, adding an additive to create a liquid phase during sintering might also help close the pores.
The discussion of the results obtained, in terms of bending properties (Figure 7) and wear behavior (Figure 8), is directly related to the microstructure of each composite. In many metal powder mixtures, fluxes or elements that form eutectics with the base metal—melting at a lower temperature—are often incorporated. In this case, however, with ceramics and aluminum, the melting temperature of aluminum cannot be reached, and ceramics have much higher melting points. The only viable alternative is to form eutectics/intermetallics with lower melting temperature, which could help close the porosity and enhance cohesion between the aluminum particles.
All pigments used have the potential to form intermetallics with aluminum, except for Limonite ochre. This is because pigments such as Mars red and BayFe red contain iron oxides, Cobalt aluminate is present in Cobalt blue, and chromium oxide in Chromium green, whereas Limonite ochre is composed of aluminum silicate with iron ore. The formation of these intermetallics may be key to understanding the mechanical properties of these composites. As mentioned in the introduction, intermetallics increase brittleness, as they suffer from poor fracture toughness at low temperatures, where cracks form more easily due to inelastic deformation [47].
The Limonite ochre pigment consists of aluminum silicate and iron ore (rich in iron oxides), while Mars red and BayFe red pigments are composed of Fe2O3 (iron oxide). Iron oxides are highly stable, so oxides would need to be reduced to metallic iron to form intermetallics. The iron oxide is reduced by Al at approximately 550 °C and subsequently, Al melts due to the large reaction enthalpy of the aluminothermic reduction of iron oxide [61]. Previous studies with mechanically alloyed Fe/B, subsequently mixed with aluminum, have shown that Al–Fe intermetallics form when sintering occurs above 700 °C, with their percentage increasing as the temperature rises [62]. Therefore, the formation of intermetallics in composites containing these pigments is not favored under typical conditions. If reduction were to occur, the material would densify, but it does not.
Figure 10a shows a general view of the microstructure of the Al–Limonite ochre composite, and Figure 10b presents a more enlarged area. In this microstructure, distinct regions can be observed. The lighter areas correspond to iron carbonates and sulfates (indicated by red squares in Figure 10b), primarily located at the aluminum grain boundaries (grey areas in Figure 10a,b). No diffusion of iron into the aluminum occurs, resulting in reduced material strength (Figure 7a). Additionally, the lack of bonding between the aluminum grains leads to porosity (highlighted by blue circles in Figure 10b), contributing to reduced ductility (Figure 7b), lower strength, high bending modulus, slightly decreased density (Figure 6a), and wear resistance (Figure 8a).
In the case of the Al–Mars red composite (Figure 11a), the larger, lighter areas correspond to iron oxides (highlighted by red squares), as well as rounded porosity, which is typical of powder metallurgical materials. A more detailed micrograph (Figure 11b) reveals not only the porosity and iron oxides, but also a slight diffusion between iron and aluminum. This diffusion can be observed within the aluminum particles, as small, lighter areas may correspond to Al/Fe intermetallics (marked by a -purple triangle). The low sintering temperature does not promote the growth of these small intermetallic particles. The stronger bond between the aluminum particles and the pigment results in bending strength (Figure 7a) comparable to that of pure aluminum, though ductility is reduced (Figure 7b). Despite this, the composite exhibits better wear resistance than the Al–Limonite ochre composite (Figure 8a). The formation of these intermetallics requires further study with EDX analysis.
Figure 12 shows the distribution of the different elements observed by EDX (X-ray microanalysis) for Al–Mars red composite. Figure 12a is a general view of an area on which the mapping shown in Figure 12b is performed. Figure 12c–e corresponds to a mapping for each detected element: oxygen, aluminum, and iron, respectively. Accumulations of oxygen and iron are observed mainly at the aluminum grain boundaries (Figure 12b–e), which correspond to the Mars red pigment. Furthermore, in Figure 12c, oxygen is also visible within the aluminum grains, and in Figure 12e iron appears inside the aluminum grains. This indicates a diffusion process of both oxygen and iron into the aluminum. The total composition of the zone by weight is 20.6% oxygen, 57.7% aluminum, and 21.6% iron. This corresponds to 33.8%, 56.1%, and 10.1% in atomic weight, respectively.
A more detailed EDX analysis was conducted within a lighter-colored region in the aluminum. Figure 13a indicates the area analyzed, and Figure 13b shows the corresponding spectrum. The weight percentages of oxygen, aluminum, and iron were 10.4%, 86.0%, and 5.6%, respectively. Additionally, five EDX analyses were conducted in different areas, all of which showed similar composition values in these lighter zones within the grain.
In the microstructure of the Al–BayFe red composite (Figure 14a), the lighter areas consist of Fe (71.86%), O (23.51%), Al (1.66%), Si (2.10%), and Na (0.875%), as shown in Figure 14b (marked with an red square). The darker areas represent the aluminum grains (Figure 14a,b). As in previous cases, no diffusion of iron into the aluminum is observed, and no Al/Fe intermetallic particles were found. Additionally, significant porosity is visible at the grain boundaries (marked with blue circles in Figure 14b), which explains its lower strength compared to the other composites, as well as its poor wear resistance.
A more thorough EDX analysis was performed on the Al–BayFe composite. Figure 15 shows the element mapping in the analyzed area, highlighted in Figure 15a. The general mapping (Figure 15b) reveals the distribution of four elements: oxygen, aluminum, iron, and barium, with individual mappings shown in Figure 15c–f. In this composite, both oxygen and iron diffuse, with barium also exhibiting diffusion. This results in BayFe red pigment accumulation along the aluminum grain boundaries (Figure 15b). Figure 15c shows that oxygen is also present within the aluminum grains. Barium appears uniformly distributed throughout the area (Figure 15e), while iron concentrates mainly in the pigment area, with some points observed within the aluminum grains (Figure 15f). The total composition by weight of the analyzed area is 18.2% oxygen, 63.6% aluminum, 17.9% iron, and 0.2% barium, which corresponds to atomic weight percentages of 29.8%, 61.8%, 8.4%, and 0.0%, respectively.
In the Al–BayFe red composite, a more detailed analysis of the brightest spots within an aluminum grain (Figure 16a) reveals the presence of oxygen, aluminum, and iron in the spectrum, but no barium (Figure 16b). However, these bright spots vary in composition. The brightest areas, such as the one marked with a red dot in Figure 16a, are richer in iron, with a composition by weight of 9.5% iron, 13% oxygen, and 77.5% aluminum. In contrast, the point marked with a blue arrow (with a grayer hue) has a composition by weight of 6.2% oxygen, 92.9% aluminum, and 0.9% iron.
Al/Co and Al/Cr intermetallics form readily at the sintering temperature, explaining their presence in the Al–Cobalt blue and Al–Chrome green composites, respectively [63,64]. The microstructure of the Al–Cobalt blue composite reveals a chemical reaction between aluminum and cobalt, leading to the formation of Al/Co intermetallics with some oxygen content (lighter areas marked by a purple triangle in Figure 17a). However, there is a lack of cohesion between the grains, along with significant porosity (Figure 17a). Figure 17b shows a more detailed view of an intermetallic zone at higher magnification. In this zone, small grey areas are intermixed with whitish areas, indicating different compositions of the intermetallic formed. The whiter areas have a higher cobalt content, while the greyer areas contain less. This preliminary analysis should be confirmed by further EDX analysis.
Figure 18 presents the general analysis of the Al–Cobalt blue composite, highlighting the presence of chromium, although this pigment is primarily a cobalt aluminate (CoAl2O4) or spinel of cobalt and aluminum. The analyzed area is indicated in Figure 18a, with the corresponding element mapping shown in Figure 18b. As observed in previous composites, the pigment is located along the grain boundaries of the matrix, occasionally preventing complete grain fusion. Oxygen is primarily concentrated in the pigment areas (Figure 18c), although some oxygen is also present within the aluminum matrix (Figure 18d). Chromium is distributed throughout the area, with slightly higher concentrations in specific pigment regions (Figure 18e). Finally, cobalt shows a distribution pattern similar to that of oxygen, being more concentrated in the pigment while also present within the aluminum grains. The overall composition of the analyzed area by weight is 34.0% oxygen, 39.4% aluminum, 6.3% chromium, and 19.0% cobalt, corresponding to atomic weight percentages of 52.4% oxygen, 36.1% aluminum, 3.0% chromium, and 8.0% cobalt.
At higher magnification of Al–Cobalt blue, particles (lighter areas) are observed within the aluminum grains (Figure 19a). The EDX spectrum shown in Figure 19b corresponds to the particle highlighted with a red rectangle in Figure 19a. This particle’s composition by weight is 11.9% oxygen, 82.8% aluminum, 2.4% chromium, and 2.9% cobalt, corresponding to atomic weight percentages of 19.0% oxygen, 78.6% aluminum, and 1.2% each for chromium and cobalt. Given the general composition of 3.0% chromium and 8.0% cobalt, this suggests a greater diffusion of chromium than cobalt. Other analyzed points contain no chromium, but maintain similar levels of oxygen and cobalt. Additionally, analysis of the aluminum matrix reveals a composition of 5.3% oxygen and 94.7% aluminum by weight. This indicates that, although oxygen content varies, oxygen is present throughout. This observation applies to all studied composites.
The Al–Chrome green composite contains aluminum, oxygen, chromium, and iron as constituent elements (Figure 20). The general analyzed area is shown in Figure 20a, with corresponding element mapping in Figure 20b. The pigment is located along the grain boundaries. Oxygen (Figure 20c) and chromium (Figure 20e) show similar distributions, both at the grain boundaries and within the aluminum grains. However, iron is distributed more evenly across the entire analyzed surface. The composition by weight is 15% oxygen, 20.6% aluminum, 13.9% chromium, and 0.5% iron, with atomic weight percentages of 24.4% oxygen, 68.3% aluminum, 7.0% chromium, and 0.2% iron. This suggests diffusion of both chromium and iron throughout the matrix.
The particles analyzed by EDX (indicated by the red dot in Figure 21a) contain 15.3% oxygen, 74.7% aluminum, 8.7% chromium, and 1.3% iron by weight. The spectrum in Figure 21b confirms these elements, with atomic weight percentages of 24.4% oxygen, 70.7% aluminum, 4.3% chromium, and 0.6% iron. However, not all particles have the same composition; other analyzed particles do not contain iron, and chromium levels vary between 9.4% and 12.1%.
According to the EDX analyses performed on all composites, particles within the aluminum grains contain oxygen in a range between 12% and 15% by weight (19% to 24% in atomic weight). This oxygen may partly result from surface oxidation of the aluminum, but a significant portion likely originates from the pigment. Thermodynamic principles in metallurgy indicate that aluminum oxidizes in the presence of chromium, cobalt, and iron oxides, and at 600 °C, an aluminothermic reaction occurs. Thus, an increase in aluminum oxides is expected. Additionally, due to the excess aluminum, it may form intermetallic compounds with Cr, Co, or Fe, as suggested by phase diagrams, although these intermetallics can be masked by alumina formed in the same region.
X-ray diffraction (Figure 22) provided information on the presence of AlCo and AlCr in small quantities, as well as chromium oxide (Cr2O3) and cobalt–aluminum spinel (CoAl2O4). The peaks are relatively small due to the predominance of aluminum. Some of the aluminum peaks coincide with those of Al–Chrome green, where at around 65°, Cr2O3 overlaps with aluminum, and at 78°, the aluminum peak slightly overlaps with the intermetallic AlCr. This is also observed in Al–Cobalt blue, where three aluminum peaks (44°, 65°, and 78°) overlap with the intermetallic phase, and the 65° peak coincides with the CoAl2O4 spinel. In any case, these peaks can be attributed to these compounds because there are other peaks that do not coincide. This indicates that the pigment has diffused into the aluminum matrix during the sintering process.
The presence of intermetallic compounds contributes to increased rigidity, while alumina reduces resistance. Combined with the pigments located at the grain boundaries, these factors result in some composites exhibiting notably low resistance.
The mechanical properties are consistent with the formation of the Al/Co intermetallics, exhibiting similar bending strength (Figure 7a), reduced ductility (Figure 7b), and a higher flexural modulus compared to aluminum. The wear behavior is also comparable to that of aluminum, though slightly higher (Figure 8a). Wear may be increased due to the porosity present in the structure.
In the Al–Chrome green composite, Al/Cr intermetallics with different stoichiometries are also formed. Qualitatively, one can observe areas with varying aluminum content, ranging from very little to almost 50%, primarily in the light regions (marked by a purple triangle in Figure 17c,d, and Figure 22). The dark areas consistently correspond to aluminum. Additionally, significant porosity can be observed (marked by blue circles in Figure 17c,d).
Despite the formation of the Al/Cr intermetallic, the properties differ from those of Al–Cobalt blue composite. It exhibits lower bending resistance (Figure 7a), higher ductility (Figure 7b), and lower bending modulus compared to Al–Cobalt blue. The wear behavior (Figure 8a) is more similar to that of aluminum, though slightly worse than that of Al–Cobalt blue.
Regarding the wear tracks (Figure 9), different areas were found: abrasive lines (indicated by blue squares) and accumulation areas (indicated by red squares). Figure 23 shows these different areas for aluminum and some composites with more detail.
As discussed in Section 2.4.2, the friction coefficient (FC) is measured as the mean value in the stable region of the FC versus slip curve. However, this stabilization is not fully achieved due to the presence of particles in the track. In this case, since the particles are pigments, they are not highly abrasive and may even slightly lubricate the track, similar to the behavior of silicates or aluminates. However, iron oxides can act as abrasive particles. These particles, referred to as the third body [65], can contribute to increased abrasion. The presence of these particles in the track can cause material deformation and retain more debris and dust. This results in the formation of wedge-shaped agglomerates at the end of the test. Figure 23a shows these agglomerates in the aluminum (highlighted with red circles) alongside an abrasive line (indicated by blue arrows). These agglomerates did not detach during wear because they were well-anchored to the matrix. These agglomerates, or buildups, contributed to adhesive wear [66]. Additionally, since aluminum is a ductile metal, it can deform, leading to a mechanism dominated by plastic deformation rather than abrasion, as the abrasive lines are not sharply defined (blue arrow in Figure 23a).
On the other hand, when the Limonite ochre pigment was introduced, the friction coefficient (FC) decreased, but the wear on the material increased (Figure 8). In this case, the pigment particles were not bonded to the aluminum matrix, so they did not form wedge-like agglomerates. Instead, debris dust was released from the track or accumulated at the edges, as shown in Figure 23b (highlighted with a black rectangle). This resulted in a wider track compared to that of aluminum. Additionally, being a silicate, the pigment may have provided slight lubrication to the track, further reducing abrasion, although some abrasive lines were observed (Figure 23b, highlighted with a blue arrow).
In terms of wear mechanisms, the two composites that most closely resemble the aluminum matrix are Al–Cobalt blue and Al–Chrome green, both exhibiting low wear values (Figure 8a). In the case of Al–Chrome green, the FC is lower than that of aluminum (Figure 8b), possibly due to the lubricating effect of the pigment. The wear tracks of both composites (Figure 23c,d) closely resemble that of aluminum in Figure 23a. They feature adhesive zones with wedge-like agglomerations (marked by red circles) and abrasive or plastic deformation zones (indicated by blue arrows)
The other two composites, Al–Mars red and Al–BayFe, exhibited wear tracks that were intermediate between those of Al–Limonite ochre and Al–Cobalt blue/Al–Chrome green. They displayed both abrasive and adhesive mechanisms, forming wedge-like agglomerations; however, in this case, the agglomerations were cracked and easily detached. As a result, these composites experienced higher wear and showed a highly variable FC across different samples.
To ensure the dimensional stability of the aluminum composites, thermal cycles were performed on the two selected materials (Al–Chrome green and Al–Cobalt blue). None of the measured variables (weight, thickness, width, length of the pieces, and color) changed during the 100 cycles performed, as shown in Figure 24a. This is expected for aluminum burners used in gas stoves (Figure 24b), which have been and will continue to be used in many households.
The lower resistance of the selected composite materials can be improved through processes that close the porosity, such as HIP, hot compaction, or double-punch compaction, as previously explained. However, the future direction, as mentioned in the introduction, is to manufacture these parts using additive manufacturing, which, once it has been demonstrated that the color can be achieved, could result in parts with no porosity and, therefore, greater resistance.
5. Conclusions
The CIELAB system, through the use of the ΔE* parameter, has proven to be an effective tool for evaluating chromatic differences between standard and test samples. Findings indicate that Cobalt blue and Chrome green exhibit ΔE* values nearing the threshold of 20, while red pigments approach 30. Limonite ochre is excluded due to its negligible color difference.
During the evaluation process, aluminum composites with inorganic natural pigments were assessed based on their visual appearance after sintering and their mechanical properties. Composites that demonstrated a balance between color fidelity, flexural strength, and tribological performance were prioritized, although some trade-offs in ductility were anticipated due to the lubricating nature of the ceramic pigments. This selection criterion led to the identification of two optimal composites for further development.
Thus, Al–Cobalt blue, in relation to the reference aluminum, has a 93% relative density, a 5% increase in bending strength, 90% less deformation, which makes the modulus increase by 160%, the wear resistance decreases by 50%, and the coefficient of friction increase by 9%. The other composite that maintains the color, Al–Chrome green, has a density of 95%; its bending strength decreases by 30%, the deformation decreases by 72%, the bending modulus increases by 77%, while the wear resistance increases by 1%, and the coefficient of friction decreases by 8%.
Ultimately, out of the five initial composites, only two compositions (Al–Cobalt blue and Al–Chrome green) were selected as viable options for manufacturing gas cooker burners using powder metallurgy. Both EDX and X-ray analysis confirmed the formation of small intermetallic particles. In addition, both composites remain stable at temperatures of up to 300 °C over the 160 cycles studied.
Conceptualization, M.A.M. and J.A.; methodology, M.A.M. and J.A.; validation, M.A.M. and J.A.; formal analysis, M.A.M. and J.A.; investigation, M.A.M. and J.A.; resources, M.A.M. and J.A.; data curation, M.A.M. and J.A.; writing—original draft preparation, J.A.; writing—review and editing, M.A.M. and J.A.; visualization, M.A.M. and J.A.; supervision, M.A.M. and J.A.; project administration, M.A.M.; funding acquisition, M.A.M. All authors have read and agreed to the published version of the manuscript.
The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author/s.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Figure 2. Inorganic oxide pigments: (a) Limonite ochre, (b) Limonite ochre after heating, (c) Mars red, (d) BayFe red, (e) Cobalt blue, and (f) Chromium green. Images taken with a mobile phone camera.
Enlarge this image.Figure 3. Representation in the CIELab system. This system consists of two axes, a* and b*, which intersect at a right angle and define the hue of the color. The third axis, L*, indicates lightness. This axis is oriented vertically in relation to the ab plane. C* is the intensity of the color and h corresponds to the actual chromaticity. Another important parameter is the difference in visual sensation, represented by ΔE*. To quantify the chromatic difference between two stimuli, the Euclidean (or Pythagorean) distance ΔE* is calculated, representing the distance between two points in a three-dimensional space. In this context, ΔE* is the hypotenuse of a triangle, with ΔL* and ΔC* as the legs. The ΔE* values obtained for a sample, compared to a reference standard, indicate whether an observer can detect any color difference. A ΔE* value of 1 indicates that the color is identical, with no visible variation between the measured and displayed colors. A ΔE* value less than 3 is generally imperceptible to the human eye.
Enlarge this image.Figure 4. Initial choice of oxides as pigments: (a) before the heating process and (b) after the heating process at 630 °C for 1 h. Images taken with a mobile phone camera.
Enlarge this image.Figure 5. Test specimens sintered at 610 °C for 30 min in a nitrogen atmosphere. (a) Al, (b) Al–pre-treated Limonite ochre, (c) Al–Mars red, (d) Al–BayFe red, (e) Al–Cobalt blue, and (f) Al–Chrome green. Macrographs taken with a mobile phone camera.
Enlarge this image.Figure 6. Physical properties: (a) density, (b) relative density, and (c) dimensional variation.
Enlarge this image.Figure 7. Bending properties: (a) bending strength, (b) elongation, and (c) bending modulus.
Enlarge this image.Figure 9. Wear tracks: (a) aluminum, (b) Al–Limonite ochre, (c) Al–Mars red, (d) Al–BayFe red, (e) Al–Cobalt blue, and (f) Al–Chrome green. Squares with blue line—abrasive lines. Squares with red lines—adhesive areas.
Enlarge this image.Figure 10. Microstructures of the Al–Limonite ochre composite: (a) general view and (b) detail at higher magnification. Squares with blue line—porosity. Squares with red line—pigment grains.
Enlarge this image.Figure 11. Microstructures of the composite Al–Mars red: (a) general view and (b) detail at higher magnification. Squares with blue line—porosity. Squares with red lines—pigment grains. Triangles with purple lines—small particles of intermetallic Al/Fe.
Enlarge this image.Figure 12. Microstructures of the composite Al–Mars red: (a) micrographic analysis of different elements; (b) mapping on the micrographic (a); (c) distribution of oxygen; (d) distribution of aluminum; and (e) distribution of iron.
Enlarge this image.Figure 13. (a) EDX analysis from Al–Mars red (red point marked the analysis area), and (b) its corresponding spectrum.
Enlarge this image.Figure 14. Microstructures of the Al–BayFe red composite: (a) general view and (b) detail at higher magnification. Squares with blue line—porosity. Squares with red line—pigment grains. Extended area has been marked in red.
Enlarge this image.Figure 15. Microstructures of the Al–BayFe red composite: (a) micrographic analysis of different elements; (b) mapping on the micrographic (a); (c) distribution of oxygen; (d) distribution of aluminum; (e) distribution of barium; and (f) distribution of iron.
Enlarge this image.Figure 16. (a) EDX analysis from Al–BayFe red (red point marked the analysis area) and (b) its corresponding spectrum. The point marked with a blue arrow corresponds to a different composition poor in iron.
Enlarge this image.Figure 17. Microstructures of the Al–Cobalt blue composite: (a) general view and (b) detail at higher magnification of intermetallic structure. Extended area marked in red, and blue circles are porosity. Triangle purple—particles of Al/Co intermetallic. Microstructures of the Al–Chrome green composite: (c) general view and (d) detail at higher magnification of intermetallic structure or Al/Cr. Extended area marked in red, and blue circles are porosity. Triangle purple—particles of Al/Co intermetallic.
Enlarge this image.Figure 18. Microstructures of the Al–Cobalt blue composite: (a) micrographic analysis of different elements; (b) mapping on the micrographic (a); (c) distribution of oxygen; (d) distribution of aluminum; (e) distribution of chromium; and (f) distribution of cobalt.
Enlarge this image.Figure 19. (a) EDX analysis from Al–Cobalt blue (red point marked the analysis area) and (b) its corresponding spectrum.
Enlarge this image.Figure 20. Microstructures of the Al–Chromium green composite: (a) micrographic analysis of different elements; (b) mapping on the micrographic (a); (c) distribution of oxygen; (d) distribution of aluminum; (e) distribution of chromium; and (f) distribution of iron.
Enlarge this image.Figure 21. (a) EDX analysis from Al–Chrome green (red point marked the analysis area) and (b) its corresponding spectrum.
Enlarge this image.Figure 22. X-ray diffraction of the Al–Chrome green and Al–Cobalt blue composites. The different peaks are marked with a violet square for the aluminum standard (JCPDS file n°: 00-004-3492), a green triangle for the AlCr standard (JCPDS file n°: 03-065-2640), and a blue triangle for the AlCo standard (JCPDS file n°: 03-065-0672). The chromium oxide standard is marked with a green circle (JCPDS file n°: 00-006-0504) and the cobalt–aluminum spinel standard is marked with a blue circle (JCPDS file n°: 01-082-2239), as indicated in the figure legend.
Enlarge this image.Figure 23. Micrograph of wear tracks: (a) aluminum track; (b) detail of the Al–Limonite ochre track: debris powder (black square) and abrasive wear (blue arrow); (c) detail of the Al–Cobalt blue track: agglomeration area (red circle) and abrasion lines (blue arrow), (d) detail of the Al–Chrome green track: agglomeration area (red circle) and abrasion lines (blue arrow).
Enlarge this image.Figure 24. (a) Thermal cycles for Al–Chrome green (green lines) and Al–Cobalt blue (blue lines), variation in weight, dimensions, and ΔE*; (b) aluminum burner in gas cookers.
Measured CIELab parameters with calculated saturation and chromaticity for sintering samples.
| Composite | L* | a* | b* | C* | h |
|---|---|---|---|---|---|
| Al | 79.86 | 0.23 | 8.20 | 1.91 | 1.54 |
| Al–Limonite ochre | 66.84 | 2.46 | 6.97 | 17.10 | 1.23 |
| Al–Mars red | 49.13 | 10.83 | 12.04 | 130.34 | 0.84 |
| Al–BayFe red | 69.81 | −3.62 | −6.96 | 25.20 | 1.09 |
| Al–Cobalt blue | 60.38 | −2.97 | 8.68 | 25.74 | −1.24 |
| Al–Chrome green | 72.96 | 1.07 | 2.55 | 2.73 | 1.17 |
Differences in color parameters between sintered aluminum and composites.
| Composite | ΔL* | Δa* | Δb* | ΔE* | ΔC* | Δh |
| Al | ||||||
| Al–Limonite ochre | 13.01 | −2.22 | 1.23 | 13.26 | −15.19 | 0.31 |
| Al–Mars red | 30.73 | −10.60 | −3.84 | 32.73 | −128.43 | 0.70 |
| Al–BayFe red | 28.82 | −11.24 | −5.02 | 31.34 | −149.76 | 0.69 |
| Al–Cobalt blue | 10.05 | 3.86 | 15.16 | 18.59 | −23.30 | 0.45 |
| Al–Chrome green | 19.48 | 3.20 | −0.48 | 19.74 | −23.83 | 2.78 |
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; Abenojar, Juana 2
1 Materials Science and Engineering Department, Universidad Carlos III de Madrid, 28911 Leganés, Spain;
2 Materials Science and Engineering Department, Universidad Carlos III de Madrid, 28911 Leganés, Spain;