Effect of 1-D and 2-D carbon-based nano-reinforcements on the dry sliding-wear behaviour of 3Y-TZP ceramics

The dry sliding-wear behaviour of a zirconia doped with 3 mol% yttria (3Y-TZP) monolithic and of two 3Y-TZP composites, these latter with the same ultrafine-grained microstructure as the former but reinforced with either 1-D carbon nanofibres (CNFs) or 2-D reduced graphene oxide (rGO) nanoplatelets, was evaluated by ball-on-disk tests at moderate load, and compared critically. It was found that 3Y-TZP, 3Y-TZP/CNFs, and 3Y-TZP/rGO undergo mild wear, in the three cases by abrasion with contributions from both plastic deformation and fracture (with varying severities depending on the sample). It was also found that wear resistance does not correlate with hardness or toughness, but with whether or not there is formation of self-lubricating tribofilms on the contact surface. Specifically, once pulled-out, 2-D rGO nano-reinforcements impose solid-state lubrication that reduces the coefficient of friction (CoF), thus providing 3Y-TZP/rGO with superior wear resistance relative to both 3Y-TZP and 3Y-TZP/CNFs. 1-D CNF nano-reinforcements, however, do not form such tribofilms, or hardly do so, thus having no effect on the CoF and wear resistance. Implications are discussed of both the dimensionality of the carbon-based nano-reinforcements and the testing conditions for the microstructural design of ceramic composites for tribological applications.     

Wear behavior of graphene/alumina composite

In the present work, the dry sliding behavior of a graphene/alumina composite material was studied against alumina in air. The tests were carried out in a reciprocating wear tester with an applied load of 20 N, a sliding speed of 0.06 m s⁻¹ and a sliding distance of up to 10 km. Under the testing conditions, the graphene/ceramic composite showed approximately half the wear rate and a 10% lower friction coefficient than the monolithic alumina. It has been found that this behavior is related to the presence of graphene platelets adhered to the surface of friction that form a self-lubricating layer which provides enough lubrication in order to reduce both wear rate and friction coefficient, as compared to the alumina/alumina tribological system.     

Microstructure evolution and tribological properties of TiB2/Ni–Ta cermets

Cermets containing TiB₂ and single or mixed metals were produced by conventional hot-pressing technique at 2100 °C for 1 h. Nickel, tantalum and their mixtures were used as alloying substances to enhance the density of TiB₂ composites. The influence of metal addition on the microstructure and tribological properties were investigated. The addition of Ta powder greatly refined the microstructure of sintered samples. Similarly, the mixture of Ni and Ta metals hindered the grain growth of TiB₂ particles during the hot-pressing while the samples were sintered up to 98% of theoretical density. The wear behaviour of the composites was assessed by ball on disk tests. The wear rate against alumina counterbody varied in the range of (5.9–21.2) × 10^?6 mm³/Nm. The friction coefficient was not affected significantly by the alloying substances and only slightly increased from 0.58 for pure TiB₂ to 0.67 for samples with Ta addition.     

Tribological behaviour of powder metallurgy-processed aluminium hybrid composites with the addition of graphite solid lubricant

The tribological behaviour of powder metallurgy-processed Al 2024–5 wt% SiC–x wt% graphite (x=0, 5, and 10) hybrid composites was investigated using a pin-on-disc equipment. An orthogonal array, the signal-to-noise ratio and analysis of variance were employed to study the optimal testing parameters using Taguchi design of experiments. The analysis showed that the wear loss increased with increasing sliding distance and load but was reduced with increased graphite content. The coefficient of friction increased with increasing applied load and sliding speed. The composites with 5 wt% graphite had the lowest wear loss and coefficients of friction because of the self-lubricating effect of graphite. Conversely, due to the effect of the softness of graphite, there was an increase in wear loss and the coefficient of friction in composites with 10 wt% graphite content. The morphology of the worn-out surfaces and wear debris was examined to understand the wear mechanisms. The wear mechanism is dictated by the formation of both a delamination layer and mechanically mixed layer (MML). The overall results indicated that aluminium ceramic composites can be considered as an outstanding material where high strength and wear-resistant components are of major importance, particularly in the aerospace and automotive engineering sectors.     

Si3N4/graphene nanocomposites for tribological application in aqueous environments prepared by attritor milling and hot pressing

Advanced ceramic materials have proved their superior wear resistance as well as mechanical and chemical properties in a wide range of industrial applications. Today there are standard materials for components and tools that are exposed to severe tribological, thermal or corrosive conditions. The main aim of this work is to develop novel, highly efficient tribological systems on the basis of ceramic/graphene nanocomposites as well as to prove their superior quality and to demonstrate their suitability for technical applications e.g. for slide bearings and face seals in aqueous media. Current research in the field of ceramic nanocomposites shows that is possible to make ceramic materials with improved mechanical and tribological properties by incorporating graphene into the Si₃N₄ structure. Multilayered graphene (MLG) was prepared by attritor milling at 10 h intensive milling of few micrometer sized graphite powders. The large quantity, very cheap and quick preparation process are a main strengths of our MLG. Si₃N₄/MLG nanocomposites were prepared by attritor milling and sintered by hot pressing (HP). The Si₃N₄ ceramics were produced with 1 wt%, 3 wt%, 5 wt% and 10 wt% content of MLG. Their structure was examined by transmission electron microscopy (TEM). The tribological behavior of composites in aqueous environment was investigated and showed the decreasing character of wear at increased MLG content. This new approach is very promising, since ceramic microstructures can be designed with high toughness and provide improved wear resistance at low friction.     

Effect of CuO on self-lubricating properties of ZrO2(Y2O3)–Mo composites at high temperatures

In this paper, ZrO₂ matrix high-temperature self-lubricating composites with addition of CuO as lubricant were prepared using a hot-pressing method by tailoring the content of CuO. The wear and friction behaviour of the composites were investigated from 700 °C to 1000 °C. The composites sliding against an Al₂O₃ ceramic ball exhibited excellent self-lubricating and anti-wear properties at high temperatures. The low friction and wear mechanisms were investigated in detail.     

Wear maps of HIP sintered Si3N4/MLG nanocomposites for unlike paired tribosystems under ball-on-disc dry sliding conditions

Hot isostatically pressed monolithic and multilayer graphene (MLG) reinforced silicon nitride nanocomposites have been investigated by ball-on-disc tests under variable loading conditions. Tests were carried out at room temperature with three different normal loads (10, 40 and 80 N), and six sliding speeds (10, 20, 50, 100, 150 and 200 mm/s) without lubrication using commercial silicon carbide ball counterparts for 54 tribosystems. The aim of the research work was to construct 2D wear transition and 3D wear rate maps of the investigated ceramic composites. The 3D maps visualizing the specific wear rate and the dimensionless wear coefficient as a function of normal load and sliding speed have been completed with morphological analysis of wear tracks and identification of the dominant wear mechanisms. The presented ceramic wear maps provide useful aid for predicting the wear performance of the investigated nanocomposites under various loading conditions.     

Evaluation of mechanical reliability of zirconia-toughened alumina composites for dental implants

Zirconia-toughened alumina composites containing 0–30 vol% of 3Y-TZP were fabricated by sintering at 1600 °C for 2 h in air. The effect of the 3Y-TZP content on the mechanical properties and microstructure of the alumina ceramics was investigated. The fracture toughness and biaxial flexural strength increased as the 3Y-TZP content increased. The Young's modulus decreased with 3Y-TZP content according to the rule of mixture, while the hardness showed the contrary tendency. The Weibull modulus of the Al₂O₃ with 20 vol% 3Y-TZP composite is higher than that of alumina. The residual hoop compressive stress developed in ZTA ceramic composites probably accounts for the enhancement of strength and fracture toughness, as well as for the higher tendency of crack deflection. No monoclinic phase and strength degradation were found after low temperature degradation (LTD) testing. The excellent LTD resistance can be explained by the increased constraining force on zirconia embedded in alumina matrix.     

Tribological properties of TiB2 and TiB2–MoSi2 ceramic composites

Recently, dense monolithic TiB₂ and TiB₂–20 wt.% MoSi₂ composites with high hardness (24–26 GPa) have been processed by hot pressing. To assess the tribological potential, the present study was performed in analyzing the influence of load on the fretting wear of TiB₂ and TiB₂–MoSi₂ composites against bearing steel. Under the investigated conditions, a higher coefficient of friction (COF) of 0.5–0.6 was recorded with all the materials with a lower COF at a higher load of 10 N. Detailed microstructural investigation of the worn surfaces was carried out using SEM–EDS and XRD in order to understand the fretting wear mechanisms. Severe wear (order of 10⁻⁵ mm³/N m) was measured for the investigated materials under the selected fretting conditions with lower wear rate for TiB₂–20 wt.% MoSi₂ composite at all loads (2–10 N). While abrasive wear dominates the material removal process in the case of monolithic TiB₂, the tribochemical wear is observed to be the predominant wear mechanism for the composite.     

Evaluation of tribological behavior of B4C–hBN ceramic composites under water-lubricated condition

The aim of this study is to evaluate the influence of hexagonal boron nitride (hBN) addition on the tribological behavior of B₄C-based ceramic composites under distilled water lubrication. Water-lubricated sliding tests of hot-pressed B₄C–hBN ceramic composites with different hBN amounts against pure B₄C ceramic were carried out on a pin-disc type wear apparatus. It was found that the addition of hBN into B₄C ceramic matrix resulted in a severe decrease of the friction coefficient from 0.373 for B₄C/B₄C sliding pair to 0.005 for B₄C–20 wt% hBN/B₄C sliding pair. A B₂O₃ tribochemical film formed on the worn surface of the B₄C–hBN specimen protected both B₄C–hBN and B₄C and facilitated the frictional surfaces to smooth. Therefore, the tribological behaviors of the pairs were significantly improved. The formation process of the film and its antifriction mechanism are discussed.     

Prepare SiTiOC ceramic coatings by laser pyrolysis of titanium organosilicon compound

A SiTiOC ceramic coating with outstanding tribological performance was prepared by laser scanning the organosilicon coating with different laser power. The composition and structure of the obtained SiTiOC ceramic coatings were analyzed by scanning electron microscopy (SEM), infrared spectroscopy (FTIR), Raman spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission electron microscope (TEM). The tribological performance of the coatings was studied using a multi-functional reciprocating friction and wear tester. The results showed that the chemical structure (chemical bonding) of the coatings prepared at 0 W, 350 W, and 500 W laser powers included Si-O-Si, Si-C, and TiO₂, while that prepared at 800 W was mainly composed of amorphous SiO₂, indicating that the coating had higher ceramization. The SiTiOC ceramic coatings prepared by the present process effectively reduced the friction coefficient and wear volume of the steel substrate, which indicated that they had good anti-friction and wear resistance properties.     

Multifunctional ceramic-metal biocomposites with Zinc containing antimicrobial glass coatings

A biocompatible glassy coating with a high antimicrobial activity (>3 log of reduction) versus the gram-bacterium Escherichia coli has been obtained. The substrates are based on new zirconia ceramic matrix composites reinforced with biological tolerant metals (3Y-TZP/Ta and 3Y-TZP/Nb biocermets). Biocompatibility was studied using NIH-3T3-cells (mouse embryonic fibroblast) and >70% viability was found. These results open up the possibility of using these materials in large panoply in orthopaedics, dentistry and other hard tissue replacement applications where biofunctional, structural and antimicrobial properties are required.     

Wear damage of Si3N4-graphene nanocomposites at room and elevated temperatures

Mechanical and tribological properties of nanocomposites with silicon nitride matrix with addition of 1 and 3 wt.% of multilayered graphene (MLG) platelets were studied and compared to monolithic Si₃N₄. The wear behavior was observed by means of the ball-on-disk technique with a silicon nitride ball used as the tribological counterpart at temperatures 25 °C, 300 °C, 500 °C, and 700 °C in dry sliding. Addition of such amounts of MLG did not lower the coefficient of friction. Graphene platelets were integrated into the matrix very strongly and they did not participate in lubricating processes. The best performance at room temperature offers material with 3 wt.% graphene, which has the highest wear resistance. At medium temperatures (300 °C and 500 °C) coefficient of friction of monolithic Si₃N₄ and composite with 1%MLG reduced due to oxidation. Wear resistance at high temperatures significantly decreased, at 700 °C differences between the experimental materials disappeared and severe wear regime dominated in all cases.     

Frictional behavior and wear resistance performance of gradient cermet composite tool materials sliding against hard materials

Gradient cermet composites possessing high surface hardness, flexural strength and interface bonding strength were fabricated using vacuum hot-pressing sintering. Ball-on-disk tests were performed to investigate the tribological properties of the gradient cermet composites against 440 C stainless steel, Al₂O₃ and Si₃N₄ balls at different sliding speed and load in comparison with traditional Ti(C,N) cermets. The tribological behavior was characterized in terms of friction coefficient and wear rate. The results showed that friction coefficient was significantly dependent on the sliding speed and load when sliding against Al₂O₃ and Si₃N₄. However, there was no obvious relation between them during sliding against 440 C stainless steel due to the formation of metal adhesive layer. Gradient cermet composites exhibited a higher friction coefficient but lower wear rate than traditional Ti(C,N) cermets. The main wear mechanism of gradient cermet composites was adhesion wear during sliding against 440 C stainless steel, while abrasion wear was the predominant mechanism during sliding against Al₂O₃ and Si₃N₄. It was expected that gradient cermet composites would be excellent candidates for cutting tool materials.     

Tribological behavior of graphene nanoplatelet reinforced 3YTZP composites

The tribological behavior of graphene nanoplatelet (GNP) reinforced 3 mol% yttria tetragonal zirconia polycrystals (3YTZP) composites with different GNP content (2.5, 5 and 10 vol%) was analyzed and discussed. Their dry sliding behavior was studied using a ball-on-disk geometry with zirconia balls as counterparts, using loads between 2 and 20 N at ambient conditions and compared to the behavior of a monolithic 3YTZP ceramic used as a reference material. The composites showed lower friction coefficients and higher wear resistance than the monolithic 3YTZP. An outstanding performance was achieved at 10 N, where the friction coefficient decreased from 0.6 to 0.3 and the wear rates decreased 3 orders of magnitude in comparison with the monolithic ceramic. A layer adhered to the worn surface was found for all the composites, but it did not acted as a lubricating film. The composites with the lowest GNP content showed an overall improved tribological behavior.     

Percolative mechanism of sliding wear in alumina/zirconia composites

In the present study, the wear resistance of zirconia-toughened alumina (ZTA) composites has been investigated under dry sliding conditions with a Hertzian contact pressures from 1.1 up to 2.2 GPa and a sliding velocity of 0.15 m/s. The wear rate for composite with 22 vol.% zirconia (3Y-TZP) content at the lower contact pressures (1.1 GPa), was found to be around two orders of magnitude higher than the ones corresponding to 14 and 7 vol.% 3Y-TZP content. This result has been explained in terms of the infinite cluster formed at the percolation threshold. Above the critical 3Y-TZP fraction of ≈16 vol.%, corresponding to the percolation threshold, a continuous path between zirconia particles takes place. In these particular cases, the wear is dominated by a percolative mechanism so that the t → m zirconia transformation creates a microcracks network, which controls the wear resistance of the composite.     

Friction and wear properties of scrap tire/potassium hexatitanate whisker composites

As a way of solving the environmental problem of waste tires, we developed a new type of friction material made of scrap tire composites with potassium hexatitanate in which rubber formed a continuous phase. The tribological behaviors of the scrap tire rubber composites were investigated by a friction and wear tester under dry conditions. According to the results, the optimum amounts of potassium hexatitanate were 5 phr in terms of the friction and wear properties up to 200 °C. The specimen containing other ingredients showed 0.72 of friction coefficient and 1.03 of wear rate which are highly compatible to those of the commercial ‘Sonata’ motor brake pad when it contains 5, 20, 10, 20, 10 phr of potassium hexatitanate, phenol, cashew, barium sulfate, and copper, respectively.     

Molecular dynamics simulation of α-ZrP/UHMWPE blend composites containing compatibilizer and its tribological behavior under seawater lubrication

In this work, blended composites with ultra-high molecular weight polyethylene (UHMWPE) as matrix polymer, α-zirconium phosphate (α-ZrP) as filler, and sodium polyacrylate (PAANa) as compatibilizer were prepared. The interfacial interaction between PAANa as a compatibilizer and the components of α-ZrP/UHMWPE was studied by molecular dynamics simulation. The friction and wear behavior of the GCr15 ball/composite friction pair under seawater lubrication under different loads were explored, and the friction and wear mechanism were analyzed. The results show that PAANa as a compatibilizer can effectively improve the interfacial interaction force between components of PAANa/α-ZrP/UHMWPE composites. The composites exhibited different trends regarding the relationship between tribological properties and α-ZrP content under various loads. The wear mechanism of composites under low load is mainly represented by extrusion deformation. With the increase of load, the wear mechanism of composites gradually changed into adhesive wear and abrasive wear (depending on the content of α-ZrP). This work provides a theoretical basis for preparing and applying other α-ZrP/polymer blend composites.     

Manipulating microstructure and mechanical properties of CuO doped 3Y-TZP nano-ceramics using spark-plasma sintering

Nano-powder composites of 3Y-TZP doped with 8 mol% CuO were processed by spark-plasma sintering (SPS). A 96% dense composite ceramic with an average grain size of 70 nm was obtained by applying the SPS process at 1100 °C and 100 MPa for 1 min. In contrast to normal, pressureless, sintering during SPS reactions between CuO and 3Y-TZP were suppressed, the CuO phase was reduced to metallic Cu, while the 3Y-TZP phase remained almost purely tetragonal. Annealing after SPS results in grain growth and tetragonal to monoclinic zirconia phase transformation. The grain size and monoclinic zirconia phase content are strongly dependent on the annealing temperature. By combining the processing techniques studied in this work, including traditional pressureless sintering, properties of the composite ceramic can be tuned via manipulation of microstructure. Tuning the mechanical properties of dense 8 mol% CuO doped 3Y-TZP composite ceramic by utilising different processing techniques is given as an example.     

Influence of the SiC grain size on the wear behaviour of Al2O3/SiC composites

Dry ceramic block-on-steel ring wear tests were performed at high loads in several Al₂O₃/20 vol.% SiC composites as a function of the SiC grain size, which ranged from 0.2 to 4.5 μm in d₅₀. The wear resistance of the monolithic alumina was radically improved by the addition of the SiC particles, reducing down to one order of magnitude wear rate. Two different behaviours were identified according to the microstructural observations on the worm surfaces: intergranular fracture and grain pull-out in the monolithic Al₂O₃, and plastic deformation and surface polishing in the composites. The wear resistance of the Al₂O₃/SiC composites increased with the SiC grain size due to their fracture toughness enhancement.