3.1. Metal Matrix Composites
Aluminum-based matrices are most commonly reinforced with various industrial waste materials and evaluated concerning their tribological performance. All of the studied metal matrix composites reinforced with industrial waste materials are presented in Table 2
Fly ash (FA)—being one of the most common industrial waste materials—is a coal combustion byproduct composed of various oxide particulates (dominated by silicon dioxide SiO2
, aluminum oxide Al2
, ferric oxide Fe2
, calcium oxide CaO, magnesium oxide MgO, potassium oxide K2
O and sodium oxide Na2
]. These oxides are the main components of coal-bearing rocks (and rocks in general). One of the first publications focused on the utilization of FA in metal matrices was an article by Uthayakumar et al. [107
]. The authors carried out multifactor-based experiments on a dry sliding wear system of stir-cast aluminum alloy 6351 with 5, 10, and 15 wt % FA reinforced composites. The authors observed that at lower loads, the sliding wear, specific wear rate, and COF were decreasing with the increasing percentage of FA. However, with the increase of the load, wear of the composite was increasing with an increase of FA content. In anther study, Dinaharan et al. [109
] estimated the wear rate of AA6061 aluminum reinforced with FA (added in the amount of 0–18 vol %) based on the experiments using pin-on-disc wear apparatus. The obtained results showed that with increasing percentage of FA in the composite, wear rate decreased from 411 × 10−5
/m at 0 vol % to 203 × 10−5
/m at 18 vol %. Simultaneously, the authors observed the improvement in microhardness of the developed aluminum matrix composites. Rani Panda et al. [108
] investigated the influence of FA (15 vol %) addition on the wear resistance of aluminum-silicon metal matrix. The authors obtained the best results for variants reinforced with plasma-treated FA (caused by the in situ conversion of SiO2
to hard SiC particles). In the study by Krushna et al. [56
], Al6061 aluminum alloy was reinforced with up to 12 wt % of FA. The results showed that the specific wear rate of the FA-based composite was always lower than pure metal alloy and alloy reinforced with identical wt % of RHA. One of the FA components—fly ash cenospheres—was utilized by Bera and Acharya, who investigated its influence (0–12.5 wt %) on abrasive wear behavior of LM6 aluminum alloy composite [111
]. Their results showed that, independently of the applied load and sliding distance, the composite with 10 wt % of fly ash cenospheres was superior to other variants in terms of the quantified wear rate. Furthermore, bottom ash—the heavier ash fraction obtained in coal combustion plants that does not rise up with flue gases—was investigated in terms of its application in forming aluminum composites (0–10 wt %) [118
]. The obtained results suggested that the values of wear rate and COFs were similar among all of the studied variants (including pure aluminum) up to 40 N of load. Exceeding the 40 N load, the composites containing 5 and 10 wt % of bottom ash exhibited higher wear rates and lower COFs than pure aluminum. The authors indicated, however, that adhesive wear was dominant for pure aluminum, whereas abrasive wear was most important for the formed composites. FA as a waste product is often used as a co-reinforcement combined with other conventional and waste additives. Patil et al. investigated the impact of combined FA/SiC on the 7075-T651 aluminum alloy-based hybrid composites [110
]. Both reinforcements were added in different SiC:FA ratios (from 60:40 to 90:10) and different volume percentages of combined additives (from 4 to 12 vol %). The lowest wear rate was observed for samples with a SiC/FA ratio of 75:25 and a total reinforcement volume of 8%. At the same time, maximal microhardness was achieved for the ratio of 90:10 and a total reinforcement volume of 12%. The authors indicated that the improvement in microhardness and wear behavior was especially evident in samples with maximum SiC content and for ratios with less than 20% of FA in the mixture of reinforcements. In the study by Kumar et al., two industrial waste materials—FA and RM—were used simultaneously to prepare an as-cast A356 aluminum alloy-based hybrid surface composites (with equal volume percentages of both wastes reaching 9%) using friction stir processing [114
]. The results suggested that the presence of reinforcements improved the microhardness and wear resistance of the aluminum alloy.
Red mud (RM) is an alkaline industrial waste generated in vast amounts during the production of alumina. Its red color is caused by the main constituent of RM—iron oxide (III) Fe2
, which makes up ca. 30–50% of its mass [113
]. The utilization of RM for the production of metal matrix composites with enhanced tribological properties was an object of interest of two studies. Singla et al. demonstrated that in the case of 6061 aluminum alloy-based composites RM waste can successfully replace expensive conventional reinforcement materials, such as SiC and alumina [113
]. Although the wear rate of RM-reinforced composites was slightly lower than SiC-reinforced composites, the values were comparable to those of Al2
-reinforced composites. The lowest wear rate for RM-reinforced composites was obtained for 7.5 wt % of RM. In another study, Devi Chinta et al. investigated the aluminum-based hybrid composites containing constant weight percentages of tungsten carbide (4%) and increasing percentages of RM (2–6%) [112
]. The obtained results confirmed that the highest wear resistance could be obtained for hybrid-composites containing 6 wt % of RM (regardless of the used particle size).
Ceramic wastes can also be a useful source of reinforcing materials. Waste porcelain ceramic particulate was used with the combination of constant weight percentage of B4
C to produce AA7075 hybrid aluminum composite [124
]. The authors showed a reduction in wear loss and values of COF with porcelain content increments up to a critical value (12 wt %), after which it began to increase. In another study, Zheng et al. demonstrated that aluminum-based hybrid composite containing both SiC (10 wt %) and ceramic waste (20 wt %) exhibited the highest COF (higher than pure aluminum alloy and SiC only reinforced composite) comparable with those obtained for conventional brake disc material [123
]. The wear rate was, however, much lower than the conventional brake disc regardless of the applied load.
Another interesting group of industrial wastes are slags—byproducts generating during metal smelting. Slags contain various metal oxides (mainly Fe2
and silicon dioxide SiO2
). Prabhakaran and Arul characterized Lm6 aluminum alloy reinforced with copper slag (up to 10 wt %) [115
]. Their results indicated that the lowest wear rate and the highest Vickers hardness were obtained for composites with maximal used percentage weight of copper slag. In another study, Sridhar Raja et al. investigated the tribological performance of A356 aluminum alloy composite reinforced with waste steel slag (0–12 wt %) [119
]. The authors observed that the weight loss due to wearing was reduced gradually with an increasing weight percentage of steel slag particles and was minimal for composite with 12 wt % of the waste.
The processing of quarry rocks generates wet grinder stone dust particles that can be used to reinforce aluminum metal matrix composites, as shown in Xavier and Suresh [120
]. The authors showed that the highest studied amount of waste addition (20 wt %) was characterized by a maximal wear resistance and hardness. Another interesting waste obtained during the processing of rocks is crushed rock sand, which is produced while crushing the rock into pieces for construction purposes. Ashok Kumar and Devaraju investigated the high-temperature wear behavior of 7075 aluminum alloy-based composites reinforced with crushed rock sand and SiC (separately and together) [117
]. The best results were obtained for a hybrid composite containing a maximal amount of SiC (i.e., 6 wt %) and 3 wt % of crushed rock sand (the maximal percentage was 6 wt %). However, the same variant also exhibited the highest COF, which is desired in terms of its application as a material used in the brake drum. Granite particulate—a waste generated while cutting granite stones—was an object of interest of Satyanarayana et al., who investigated its impact on the tribological performance of the aluminum-silicon alloy A356 (also with co-addition of graphite) [121
]. The obtained results suggested that either reinforcement with sole granite (2 wt %) or with a combination of granite and graphite (4 and 2 wt %, respectively) contribute to a decrease in the value of COF as compared to neat A356 alloy. In another study, a limestone slurry powder (also a waste from the stone cutting industry) was used to reinforce the aluminum-magnesium-silicon alloy matrix [116
]. The authors observed a decrease in COFs for samples reinforced with the limestone slurry powder, with the optimal weight percentage of waste being 12% (the maximal studied content of waste was 16 wt %).
Krishnan et al. study was focused not only on the utilization of the waste as a reinforcement but also as a matrix [122
]. The authors used scrap aluminum as a matrix and spent alumina catalyst from the oil industry as a reinforcement (5 wt %). The produced composites were compared with other variants containing either AlSi7Mg alloy or alumina Al2
. The authors observed the highest weight loss in the abrasive test for a composite consisting of both wastes, whereas the lowest weight loss was demonstrated by a composite composed of scrap aluminum waste and alumina.
Although the majority of the studies were focused on pure aluminum or aluminum-based alloys, also composites based on other metals were tested concerning their tribological performance. For example, in the study by Zikin et al., recycled tungsten carbide (WC) and cobalt (Co) hard metal powder (originating from hard metal scarp) was used as a coating of Castolin nickel-based powder and compared with the nickel-based reference hardfacing consisting of 40 vol % of tungsten carbide [125
]. The results showed that the produced coating was less resistant to abrasive wear (by a factor of two) than conventionally used reference containing tungsten carbide. In another study, silicon bronze alloy was reinforced with marble dust particulates (0–10 wt %) [126
]. The experiment exhibited that depending on the studied tribological parameter (i.e., sliding velocity or normal load), the values of specific wear rate and COF differed among the variants. The lowest specific rate (while studying sliding velocity) reached the lowest value for the composite with 10 wt % of marble dust. On the other hand, while investigating the normal load influence on specific wear rate, the lowest wear rate was observed for the composite with 2.5 wt % of marble dust. Marble dust (1.5–6 wt %) reinforced copper alloy was also used by Rajak et al. [127
]. The authors observed that all composites were characterized by lower wear loss against different sliding velocities compared to copper alloy without reinforcement. However, the best antiwear resistance was observed for 4.5 wt % of reinforcement. In addition, COF was the highest in the case of a sample with 4.5 wt % of reinforcement and the lowest for copper alloy without marble dust. Thus, the authors suggested the composites with 4.5 wt % of marble dust for bearing applications.
3.2. Polymer Matrix Composites
Industrial wastes are more commonly used as reinforcements for polymer matrices. All of the studied polymer matrix composites reinforced with industrial waste materials are presented in Table 2
FA and fly ash cenospheres were extensively studied for their application as additives to polyester, polyethylene, epoxy resin or nylon matrices. Bishoyee et al. evaluated the erosive wear rate of composites produced from polyester resin, E-glass fibers (50 wt %) and cenospheres (0–20 wt %) [94
]. The authors indicated that the variants with maximal (20 wt %) content of fly ash cenospheres exhibited the lowest erosion rate (regardless of the applied impingement angle). In another study, Chand et al. examined organosilane modified cenospheres (10–20 wt %) as a filler for an HDPE [102
]. Abrasive wear tests showed that the lowest specific wear rate, but also the highest impact strength, were obtained for composites reinforced with 10 wt % of organosilane treated fly ash cenospheres. The authors suggested that the observed increasing wear rate of composites with the increasing weight percentage of cenospheres is probably caused by the rapid chipping of the particles and matrix at the interface. The hybrid composites made of epoxy resin, bamboo fiber (33 wt %) and fly ash cenospheres (0–6 wt %) were investigated by Jena et al. [85
]. The results of erosion wear tests showed that the lowest value of erosion wear rate was observed for the composite with maximal (6 wt %) content of fly ash cenospheres. Two industrial wastes—FA and granite powder—were the subject of the study carried out by Ray et al., who used both wastes separately (0–15 wt %) to reinforce composite made of polyester resin (45–60 wt %) and glass fiber (constant 40 wt %) [95
]. Although the addition of both reinforcements significantly reduced the erosion rate of the composites, the variant with 15 wt % FA filler turned out to be the most resistant to erosion. The same authors also investigated identical composites concerning their abrasive wear resistance [93
]. In contrast to their previous results focusing on erosion resistance, the composites filled with 15 wt % granite powder showed the best abrasion resistance.
RM waste was often studied as reinforcement in polymer-based composites. Biswas and Satapathy evaluated the erosion rate of hybrid bamboo–epoxy and E-glass-epoxy (the content of either bamboo or E-glass fibers was 50 wt %) composites filled with RM in various weight percentages (0–20 wt %) [67
]. Their results showed that the increase of the RM content improved the erosion resistance of the composites, with the lowest erosion rate obtained for bamboo–epoxy composite with 20 wt % of RM. A sliding wear test for epoxy-based composites (either homogenous or graded) reinforced with 0–20 wt % of RM was carried out in Siddhartha et al. [69
]. The authors indicated that the lowest specific wear rate and the lowest COF were exhibited by homogenous composites with 10–20 wt % of RM reinforcement. Another polymer matrix—unsaturated polyester resin—reinforced with nanoparticles of RM (0–4 wt %) was studied by Suresh and Sudhakara [96
]. Based on the results of the sliding wear test, the authors observed that regardless of the applied sliding speed, the lowest wear rates and COFs were obtained for nanocomposites filled with a maximal 4 wt % of RM. In another study, Richard et al. also evaluated the dry sliding wear properties of the nanocomposites made of unsaturated polyester resin and various size nanoparticles of RM (0–2.5 wt %) [97
]. The authors obtained the lowest specific wear rate for nanocomposites reinforced with the highest content (2.5 wt %) of 110 nm-sized RM particles (i.e., particles with the smallest studied size). A hybrid composite composed of unsaturated polyester resin, pineapple fiber (non-waste material) and RM (10–20 wt %) was evaluated concerning its either sliding wear or erosion resistance by Sundarakannan et al. [98
]. The results obtained by the authors indicated that with an increasing weight percentage of RM reinforcement, the sliding wear rate of composites decreases, whereas the erosion wear rate increases, suggesting that the potential utilization of the fabricated composite may depend on its potential industrial application.
Other wastes generated during the metallurgical processing of metal ores are slags and sludges produced during iron and steel making. Linz–Donawitz (LD) slag is generated from an LD converter during the production of steel and is mainly composed of CaO, Fe, and SiO2
. Blast furnace (BF) slag is solid waste generated from blast furnace and contains various oxides, such as SiO2
, CaO, Fe2
, MgO, Al2
. The other waste is LD sludge solid waste generated during the cleaning of flue gas emerging from the LD converter. It mainly contains FeO, Fe2
and CaO [75
]. LD slag was an object of interest of several publications of Pati and Satapathy and Pati et al. [70
]. As shown by Pati and Satapathy, the reinforcement of the composites with LD slag improved the erosion wear resistance of the developed epoxy-based or hybrid glass fiber-epoxy (20 wt % of glass fiber) composites with the best erosion resistance observed for maximal weight percentage of LD slag (either 22.5 or 30 wt %). In the study by Pati et al., hybrid glass fiber-propylene composites filled with up to 22.5 wt % of reinforcement were evaluated concerning their erosion wear. The authors observed that LD slag content is a significant control factor for minimizing the erosion rate of the produced composites. Numerous publications of Purohit and Satapathy studied the influence of LD slag, LD sludge and BF slag on the sliding wear and erosion wear of the composites based on epoxy resin matrix [72
]. The obtained results indicated that with increasing content of filler, both erosion wear- and sliding wear resistance of the composites increases. Additionally, their comparative studies showed that either sliding- or erosion wear rate reached minimal values for composites filled with LD sludge, while for composites filled with LD slag and BF slag, the results were comparable. In another study, Erdoğan et al. carried out a study comparing the BF slag, converter slag and ferrochromium slag (30 wt % in each case) as a material used to reinforce epoxy-based composites [76
]. The obtained results showed that the produced slag reinforced composites generally exhibited similar or better tribological properties than alumina reinforced composites, while the highest sliding abrasion resistance was observed for composite filled with BF slag. Hybrid composites prepared by reinforcement of PP matrix with BF slag (0–30 wt %) and (alternatively) short glass fibers (0–20 wt %) were investigated by Padhi and Satapathy [88
]. The authors demonstrated that the highest erosion wear resistance was obtained for composites filled with a maximal 30 wt % of BF slag (regardless of the addition of glass fibers). The waste obtained at the final refining step of a hydrometallurgical zinc plant was evaluated as a reinforcement for difunctional epoxy monomer-based resin [86
]. The results of the study indicated that the lowest wear rate could be obtained for composites with 30 vol % of waste (the maximal studied volume of waste was 50%) and for the largest studied particle size (i.e., larger than 208 μm). Another waste generated during various processes of iron and steel forming is an iron scale composed mainly (ca. 96%) of iron oxide (III) Fe2
. This waste was utilized as a filler (0–20 wt %) in a propylene matrix by Erdogan et al. [91
]. The authors showed that the lowest COF and volume loss was exhibited by the composite variant containing 5 wt % of iron scale. The aluminum smelting process produces residue waste called white aluminum dross containing mostly alumina (Al2
). In a study by Samat et al., this type of waste was used as a reinforcement in PP-based composites with various weight percentages of the filler (0–40 wt %) [89
]. The authors indicated that the lowest wear rate was exhibited by a composite variant containing the highest amount of the dross (i.e., 40 wt %). The same aluminum dross was used to produce micro- and nanosized alumina, which was further utilized as a filler (0–7 wt %) in PP-based composites [92
]. The lowest wear rate was exhibited by the composite reinforced with 7 wt % of nanosized alumina, but each of the tested variants performed better than pure PP.
Other potentially useful waste materials are byproducts obtained during the processing of different types of rocks. Marble dust waste is generated during the processing of the marble rocks and is composed mainly of CaCO3
, CaO, MgO and other oxides (SiO2
]. Choudhary et al. investigated the influence of marble dust (0–30 wt %) and E-glass fiber mat (10 layers) on the tribological performance of epoxy-based composites [81
]. Composites with the highest marble dust content exhibited the lowest erosion rate regardless of the applied impingement angle and impact velocity. In their two literature reports, Nayak et al. determined the sliding wear behavior of hybrid composites made of unsaturated polyester resin, E-glass fiber mat (10-layers, 40 wt %) and waste marble dust (0–16 wt %) [99
]. The authors demonstrated that the highest sliding wear resistance was observed for the composites with maximal waste marble dust weight percentage (16 wt %). Granite powder—a solid waste generated from the stone processing industry—was evaluated in terms of its potential use as a reinforcement (0–15 wt %) in a hybrid composite containing epoxy resin and glass fiber (40 wt %) [82
]. The performed erosion wear tests showed that 10 wt % of filler addition was optimal concerning erosion resistance of the studied materials. Slate powder, obtained from slate rock tailings, is a mineral waste material that was used to produce hybrid composites made of phenolic resin (10–35 wt %), glass fiber (10 wt %), alumina (7.5 wt %) and graphite (7.5 wt %) [103
]. These composite formulations containing 40–65 wt % of slate powder were compared to similar composites containing barite (65 wt %). The authors observed that the best wear performance was exhibited by the variant containing 40 wt % of slate powder, which was slightly below the level of the composite produced with the use of barite. Iron mud, a major solid waste produced in iron mining and ore processing, was an object of interest in numerous studies performed by Pani et al. [63
]. This waste is composed mainly of iron oxide (III) Fe2
, aluminum oxide Al2
and silicon oxide SiO2
and became a serious threat to the soil environment due to its long-term storage. The authors demonstrated that glass fiber-epoxy composites (50 wt % of glass fiber) reinforced with 0–20 wt % of iron-mud waste could be potentially used as tribological materials. The highest abrasive wear resistance was obtained for composites reinforced with a maximal 20 wt % of iron-mud. However, regarding the erosion wear resistance, no clear relationship between the iron-mud content and erosion wear was observed as it changed unevenly with impingement angle and erodent velocity.
Boron-containing waste generated during borax production was an object of interest of two studies carried out by Uygunoglu et al. [77
]. This waste containing similar amounts of B2
, CaO and MgO was used up to 50 or 66 wt % to reinforce the epoxy resin matrix. Abrasive wear tests performed in the first study showed that the composite with maximal content of boron-containing waste (50 wt %) exhibited the lowest abrasive wear rate and COF. The second study, however, indicated that the wear length of the composites slightly increased with increasing the filler content up to 33 wt %, which was explained by the poor interfacial bonding with the epoxy matrix.
In a study by Lin and Schlarb, waste carbon fibers were used as a reinforcement (10 wt %) in polyether ether ketone (PEEK) polymer matrix, which was also filled with solid lubricant (graphite), ZnSO4
]. The obtained results revealed that composites reinforced with recycled carbon fibers exhibited similar tribological properties to the variants filled with virgin carbon fibers. In another study, Aslan et al. investigated the tribological performance of PP-based composites containing sisal fibers (up to 30 vol %, non-waste material), waste carbon fibers (up to 27 vol %) and waste E-glass fibers (up to 21 vol %) [90
]. The lowest values of COF were obtained for composites not containing sisal fibers, especially the composites reinforced with 27 vol % of waste carbon fibers. In the study of Acikbas and Yaman, waste glass fibers (5–20 wt %) and waste wall tile (40–55 wt %, a waste from wall tile factory) were used to reinforce epoxy resin matrix [84
]. The authors demonstrated that the lowest wear rate was exhibited by the fine particle composite containing 55 wt % of waste wall tile and 5 wt % of waste glass fibers.
Another waste material that caught the attention of researchers is red brick dust, a powder formed from deformed bricks in the process of their manufacturing. Pati developed a hybrid epoxy composite containing 15 wt % of short E-glass fibers and various amounts of red brick dust (0–30 wt %). The obtained materials were evaluated in erosion wear tests and exhibited better erosion wear resistance than epoxy-glass fiber composites, with the lowest erosion wear for composites filled with 30 wt % of red brick dust.
Coal mine overburden waste was an object of interest in the study performed by Das et al. [83
]. This waste material was mixed with epoxy resin matrix in various proportions (0–40 wt %) and evaluated concerning its sliding wear resistance. The obtained results indicated that the highest content of waste material (40 wt %) caused a maximal wear resistance among the studied variants.
Ruggiero et al. utilized waste glass beads—a waste material originating from glass blasting—to produce epoxy-based composites with improved tribological properties [79
]. Comparing the composites with the control and those filled with non-waste glass powder, the lowest values of COFs and the lowest wear were observed for variants containing 5–20 vol % of the largest size of waste particulate.
Another material—carbon obtained from the pyrolysis of polymer wastes—was used in the study by Myalski et al. to reinforce polyamide (PA) thermoplastic composites [105
]. The 10 wt % addition of this modified waste material, however, lead to an increase in COF as compared to neat PA variant (although the differences were not statistically significant), suggesting its poor performance as a reinforcement.
One of the oldest studies dealing with waste management for tribological purposes was a publication of Xiang and Tao, who studied mechanical and tribological properties of composites produced from polytetrafluoroethylene (PTFE) and PTFE waste (20 wt %) generated during the manufacturing of various PTFE products [104
]. The authors indicated that the COF increased with the addition of PTFE waste, but at the same time, a significant decrease in wear rate was observed. It was also observed that the addition of alumina nanoparticles (15 wt %) further improve the tribological performance of the produced materials.