Matrix Self-lubricating Composites Research Articles

Synergistic lubrication effects of Sn–Ag–Cu and MXene–Ti3C2 to improve tribological properties of M50 bearing steel with microporous channels

Purpose This paper aims to study the synergistic lubrication effects of Sn–Ag–Cu and MXene–Ti3C2 to improve the tribological properties of M50 bearing steel with microporous channels. Design/methodology/approach M50 matrix self-lubricating composites (MMSC) were designed and prepared by filling Sn–Ag–Cu and MXene–Ti3C2 in the microporous channels of M50 bearing steel. The tribology performance testing of as-prepared samples was executed with a multifunction tribometer. The optimum hole size and lubricant content, as well as self-lubricating mechanism of MMSC, were studied. Findings The tribological properties of MMSC are strongly dependent on the synergistic lubrication effect of MXene–Ti3C2 and Sn–Ag–Cu. When the hole size of microchannel is 1 mm and the content of MXene–Ti3C2 in mixed lubricant is 4 wt.%, MMSC shows the lowest friction coefficient and wear rate. The Sn–Ag–Cu and MXene–Ti3C2 are extruded from the microporous channels and spread to the friction interface, and a relatively complete lubricating film is formed at the friction interface. Meanwhile, the synergistic lubrication of Sn–Ag–Cu and MXene–Ti3C2 can improve the stability of the lubricating film, thus the excellent tribological property of MMSC is obtained. Originality/value The results help in deep understanding of the synergistic lubrication effects of Sn–Ag–Cu and MXene–Ti3C2 on the tribological properties of M50 bearing steel. This work also provides a useful reference for the tribological design of mechanical components by combining surface texture with solid lubrication. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-12-2023-0381/

The Microstructure and Mechanical Properties of High Entropy Alloy CoCrFeNiMn Matrix with Cr3C2 Reinforcement and Ag, BaF2/CaF2 Solid Lubrication

A series of CoCrFeNiMn high-entropy alloy matrix self-lubricating composites were prepared by spark plasma sintering. The composites are composed of an FCC phase, Cr7C3, Ag, and eutectic fluoride BaF2/CaF2 phases. The microstructure of the composites is uniform. The additional phases distribute along the boundary of equiaxed grains of the FCC phase. The compressive yield strength and fracture toughness decrease with the increase of eutectic fluoride BaF2/CaF2. The composites are susceptible to brittle cleavage fracture.

High temperature self-lubricating Ti-Mo-Ag composites with exceptional high mechanical strength and wear resistance

Titanium alloys are of keen interest as lightweight structural materials for aerospace and automotive industries. However, a longstanding problem for these materials is their poor tribological performances. Herein, we designed and fabricated a multiphase Ti-Mo-Ag composite (TMA) with heterogeneous triple-phase precipitation (TPP) structure by spark plasma sintering. A lamellar α-phase (αL) precipitates from the β-phase under furnace cooling conditions and maintains a Burgers orientation relationship (BOR) with β-matrix. An active eutectic transition also occurs in the titanium matrix, resulting in TiAg phase. The intersecting acicular TiAg and lamellar αL cut β grains into fine blocks and the primary equiaxed α phase also provides many interfaces with β phase, which together effectively impede dislocation movement and increase strength. Compared with other titanium composites, TMA with TPP microstructure gets an excellent combination of strength (yield strength 1205 MPa) and toughness (fracture strain 27%). Furthermore, the TPP structure endows TMA with strong cracking resistance, which aids in reducing abrasive debris at high temperatures during sliding and obtaining a low wear rate. Simultaneously, Ag particles distributed at grain boundaries will easily diffuse to the wear surface, in situ forming the necessary lubricating phase Ag2MoO4 with Mo-rich matrix debris via oxidation. TMA possesses excellent tribological properties with especially low wear rate of 8.0 × 10−6 mm3N−1m−1 and friction coefficient (CoF) of merely 0.20 at 600 °C. Unlike other self-lubricating composites with high volume fraction of soft ceramic lubricants, which inevitably sacrifice their mechanical strength and ductility, the composite TMA possesses well-balanced strength, toughness and self-lubricating properties. It holds important implications to design other metal matrix self-lubricating composites (MMSCs) used for load-bearing moving parts.

Improving the mechanical and tribological behavior of Cu-WS2 self-lubricating composite with the addition of WS2 nanosheet

Copper matrix self-lubricating composites (CMSCs) are commonly used as sliding contacts in aerospace craft for their good electrical conductivity and wear resistance. In efforts to further improve performance and prolong the service life of CMSCs, the effect of WS2 nanosheet addition ranging from 10 to 25 wt% on the mechanical and tribological properties of Cu-WS2 self-lubricating composites was analyzed in this work. The dry sliding wear of composites against beryllium copper counter discs was measured at the load of 1 N. The increase in WS2 nanosheet leads to a decline in the strength of Cu-WS2 composite, whereas it improves the tribological properties presented as the reduction of friction coefficients and wear rates. Compared to Cu-WS2 with submicron WS2 sheet, Cu-WS2 with WS2 nanosheet shows both improved mechanical and tribological properties. The wear rate of Cu-WS2 composite with 25 wt% WS2 nanosheets is 61.2% lower than that with 25 wt% submicron WS2 sheets. The addition of WS2 nanosheet reduces the size of WS2 agglomerates and generates finely dispersed WS2 particles in Cu-WS2 composites. It not only promotes lubrication film continuous and smooth, but also inhibits the initiation and propagation of cracks at the worn subsurface by improving the strength of Cu-WS2 composite.

Tribological Behavior and Self-Healing Properties of Ni3Al Matrix Self-Lubricating Composites Containing Sn-Ag-Cu and Ti3SiC2 from 20 to 800 °C

As a high-temperature structural material, Ni3Al matrix composites are often used to manufacture basic mechanical components that need to be used in high-temperature conditions. To meet the increasing demand for metal matrix composites with an excellent tribological performance over a wide temperature range, Ni3Al matrix self-lubricating composites containing Sn-Ag-Cu and Ti3SiC2 (NST) were synthesized via laser-melting deposition. Dry sliding friction tests of NST against Si3N4 ball were undertaken from 20 to 800 °C to investigate the tribological behavior and wear-triggered self-healing properties. The results show that the tribological behaviors of NST are strongly dependent on the testing temperature and self-healing properties. At low and moderate temperatures from 20 to 400 °C, as the Sn-Ag-Cu flows into the cracks and is oxidized during sliding friction, while the cracks on the worn surface are filled with oxides consisting mainly of Al2O3, SnO2 and CuO. At higher temperatures of 600 and 800 °C, the cracks are filled by the principal oxides of Al2O3, TiO2 and SiO2 due to the partial decomposition and oxidation of Ti3SiC2. Compared with other testing temperatures, the recovery ratio relative to the Ni3Al base alloy of the cracks on the worn surface of NST is the highest at 400 °C, which is about 76.4%. The synergistic action mechanisms of Sn-Ag-Cu and Ti3SiC2 on the crack self-healing from 20 to 800 °C play a significant role in forming a stable solid lubricating film, improving the anti-friction and wear resistance of NST. The results provide a solution allowing for metal matrix composites to achieve excellent lubrication stability over a wide temperature range by virtue of the crack self-healing properties.

Achieving well-balanced mechanical and tribological properties of copper matrix self-lubricating composites by adding pre-metallized B4C particles with core-shell microstructure

Copper matrix self-lubricating composites have the advantage of low friction coefficient, but usually suffer from the low mechanical strength, ductility and wear resistance. Ceramic particles reinforcing has been demonstrated an effective way in strengthening metal matrix composites. However, the poor wettability was always a critical issue. In this study, B4C particles were introduced into a copper matrix self-lubricating composite. Before spark plasma sintering, the B4C particles were pre-metallized with Ni to form a core-shell microstructure (Ni@B4C), which improves wettability and bonding strength with the copper alloy, builds up a coherent/semi-coherent interface, and notably enhances mechanical properties. After the incorporation of pre-metallized Ni@B4C particles, the compressive strength, hardness and engineering strain are increased by 62, 113 and 76%, respectively. What is more, the composite exhibits improved tribological performances. Owing to the successful transfer of lubricant WS2 to the wear surface during sliding as well as the greatly enhanced hardness, its friction coefficient is decreased to 0.16, while the wear rate is reduced to 7.53 × 10−6 mm3/N·m, which is 81% lower than that of CW. The composite achieves well-balanced mechanical and tribological properties after pre-metallized B4C particles incorporation.

Tuning the Parameters of Cu–WS2 Composite Production via Powder Metallurgy: Evaluation of the Effects on Tribological Properties

Metal matrix self-lubricating composites exhibit outstanding performance in various environments, reaching the required properties by modifying the reinforcement–matrix ratio and the production method. The present research investigated the effects on tribological performance and electrical properties of different pressure loads, maintaining pressing time, and sintering temperatures during the production of copper–10 wt% tungsten disulfide (Cu–WS2) composite via powder metallurgy. Moreover, additional thermo-mechanical treatments were evaluated, namely second pressing and second sintering steps. The density and the hardness of the produced composites were measured, as well as the electrical resistivity, considering sliding electrical contacts as possible employment. The outputs of the wear tests were considered together with the analysis of the wear track via scanning electron microscopy and confocal laser scanning microscopy to understand wear mechanisms. Different production routes were compared in terms of electrical resistivity, wear coefficient, and specific wear rate, calculated by the confocal laser scanning microscopy, and friction coefficient, measured during the wear test. The main results highlighted that the increase in sintering temperature was detrimental to the hardness and tribological properties; higher load and additional pressing step determined a general improvement in the tested properties.

Effects of graphite contents on the microstructure evolution, mechanical properties and high temperature tribological behavior of Cu–Ni–Al/Gr solid-lubricating composites

High temperature copper matrix self-lubricating composites are essential for industrial applications. This paper fabricated the Cu–Ni–Al/Gr solid-lubricating composites with 2.5–10 wt% graphite. Effects of graphite contents on the tribological behavior at room temperature to 500 ℃ were thoroughly studied. The conclusions present that the addition of graphite significantly improves tribological properties while no further enhancement can be made when graphite reaches 7.5–10 wt%, which shows the friction coefficient of 0.04–0.22 and wear rate of (1.02–10.12)× 10−5 mm3/Nm. The excellent tribological performance at 25–500 ℃ is attributed to the synergistic effects of the self-lubricating property provided by graphite lubricant and the tribo-oxide layer formed during the sliding process.

Improving high-temperature wear resistance of NiCr matrix self-lubricating composites by controlling oxidation and surface texturing

• NiCr matrix composites with WS 2 as lubricant suffer severer wear degradation at high temperatures • NiCr-WS 2 -Ti composite with heterostructure is prepared by spark plasma sintering. • The composite NiCr-WS 2 -Ti exhibits low friction coefficient and high wear resistance at elevated temperatures • Ti is preferentially oxidized, forming abundant TiO 2 protrusions and constructing in situ a special surface texture. • By controlling oxidation and surface texturing, a protective glaze layer formed at surface, leading to excellent tribological properties Self-lubricating composites (SLCs) are widely used in the fields of aerospace and marine, but the conventional NiCr matrix SLCs with sulfide as solid lubricant often suffer from low wear resistance at high temperatures. In view of its high affinity with oxygen and also the high oxidation rate, appropriate amount of nano Ti was added to NiCr-WS 2 composites prepared by spark plasma sintering (SPS) to adjust the oxidation behavior and surface texture. When exposed to high temperature, Ti was preferentially oxidized in comparison to Ni and Cr, resulting in abundant TiO 2 protrusions and microdimples on the surface, i.e. in situ surface texturing. Besides, TiO 2 was of high chemical activity and readily to react with other oxide debris during high temperature sliding process to form compounds of NiTiO 3 and CrTi 2 O 5 . The high chemical activity of oxide debris that was conducive to sintering, combining with the special surface texture that stores as many wear debris as possible, promoted the rapid formation of a protective glaze layer on the sliding surface. The NiCr-WS 2 -Ti composite exhibited low friction coefficient but high wear resistance at elevated temperatures. Especially at 800 °C, it presented a wear rate of as low as (2.1±0.3) × 10 −5 mm 3 N −1 m −1 , accounting for only 2.7% of that of NiCr-WS 2 composite.

Tribological properties of Ni3Al matrix self-lubricating composites with a gradient composite structure prepared by laser melt deposition

As the contact part of metal-matrix self-lubricating composites during sliding friction, the friction interface layer directly affects the tribological performance of the material. However, the formation of the friction interface layer with outstanding tribological performance is limited by the friction conditions during sliding friction. To address this problem, based on the antifriction and wear resistance mechanisms of the in situ formed friction interface layer of Ni3Al matrix self-lubricating composites (NMSCs) with homogeneous solid lubricant, Ni3Al matrix self-lubricating composites with a gradient composite structure (Ni3Al-GCS) were prepared via laser melt deposition, in which each component layer contained different contents of Sn-Ag-Cu and Ti3SiC2. Dry sliding friction tests of Ni3Al-GCS against GCr15 steel balls were performed under different loading conditions. The results showed that the tribological performances of Ni3Al-GCS in the range of 4–16 N were less affected by the variation of the loading conditions than those of NMSCs. The gradient composite structure of Ni3Al-GCS could reduce the dependence of the tribological behavior on the friction conditions, resulting in excellent antifriction and wear resistance of Ni3Al-GCS in a wide load range. In addition, the gradient composite structure could reduce the sliding contact damage of the friction contact surface of Ni3Al-GCS, and contribute to the formation of friction interface layer rich in the lubrication phase and oxides, thus improving the tribological performance of Ni3Al-GCS during sliding friction. This study provides new approaches for the tribological design of metal-matrix self-lubricating composites in a wide load range.

Heterostructured NiCr matrix composites with high strength and wear resistance

• NiCr-WS 2 composites with nano-Ti addition were prepared by spark plasma sintering. • The in situ reaction between NiCr, WS 2 and Ti leads to the formation of a heterostructure. • The heterostructured composite exhibits balanced mechanical and tribological properties. Self-lubricating composites are required to have excellent tribological performances and good mechanical properties when they are applied as mechanic parts. However, the brittle nature of most lubricants will weaken the mechanical properties of the composites where in order to get the lubricating ability proper amount of lubricants are usually incorporated, bringing challenges for them to get well-balanced mechanical and tribological properties. In this study, NiCr matrix self-lubricating composites with the addition of WS 2 and nano-Ti were prepared via spark plasma sintering. The hard phase of Ni 3 Ti and the lubricant TiS were formed via in-situ reaction at the interface, which surrounded the soft alloy matrix and developed a heterostructure. Compared with the conventional NiCr-WS 2 composite, the heterostructured composite exhibited a high yield strength of 1645 MPa, a low friction coefficient of 0.37 and a wear rate of as low as 2.25 × 10 −5 mm 3 N − 1 m − 1 when sliding. Finite element analysis demonstrated that the heterostructure plays a key role to balance well the mechanical and tribological properties of the NiCr matrix composites.

Physico-Mechanical Properties of Metal Matrix Self-Lubricating Composites Reinforced with Traditional and Nanometric Particles

Innovative nanostructured materials offer the possibility of enhancing the tribological performance of traditional materials like graphite and molybdenum disulfide (MoS2). In this study, the scratch resistance of two different copper powders, dendritic and spherical, and their composites with traditional MoS2, nanometric MoS2, and graphene nanoplatelets was investigated. Metal powder metallurgy was employed to produce composite materials with 5 wt% and 10 wt% of each solid lubricant. A ball milling step was employed to grind and mix the matrix copper powder with the lubricants. The use of a cold press combined with the sintering in inert atmosphere at 550 °C limited the oxidation of the copper and the degradation of the solid lubricants. The so-produced materials were characterized through a variety of techniques such as micro-indentation hardness, electrical resistivity, contact angle wettability, X-ray diffraction, Raman scattering, and scanning electron microscopy. Moreover, micro-scratch tests were performed on both pure copper and composite materials for comparing the apparent scratch hardness and friction coefficients. The scratches were examined with confocal laser scanning microscopy (CLSM), to identify the evolution of the damage mechanisms during the formation of the groove. The results highlighted the important difference between the dendritic and spherical copper powders and demonstrated a way to improve wear behavior thanks to the use of nanometric powders as solid lubricants.

High-entropy FeNiCoCr alloys with improved mechanical and tribological properties by tailoring composition and controlling oxidation

Many metal matrix self-lubricating composites possess excellent comprehensive properties of high strength and wear resistance after incorporating various lubricants that usually belong to ceramic phase. However, this improvement is always obtained at the cost of notable decrease in toughness. In order to break through this toughness-tribological properties trade-off, self-lubricating composite alloys based on the matrix of FeNiCoCr0.5 high entropy alloy (HEA) were prepared by milling addition of element Co or Cr and then spark plasma sintering (SPS). Co or Cr is added into the base HEA with the aim of tailoring oxidation behavior, mechanical properties as well as tribological performances. As no ceramic lubricants are used, the alloy composites remain high toughness. Co or Cr element addition leads to the precipitation of a hard phase of σ-Cr and grain refinement, both of which contribute to the increase of mechanical strength. On sliding, oxidation of Fe and Ni is suppressed. Instead, the oxides of cobalt and chromium are formed upon the Co- and Cr-modified HEAs, respectively. These oxidation products, especially the oxides of cobalt that are easy to be sintered on sliding, are in favor of the formation of a lubricating glaze layer at 400 °C on the worn surface, and thus the decrease in friction coefficient and wear rate, which are 0.33 and 4.0 × 10−5 mm3/(N m), respectively.

Microstructure, Tribological Behavior, and Strengthening Mechanisms of the Friction Interface Layer with Nanocrystalline Structure of Ni3Al Matrix Self-lubricating Composites

This work investigated the microstructure, tribological behavior and strengthening mechanisms of friction interface layer with nanocrystalline structure of graphene-reinforced Ni3Al matrix composites (GNMCs) synthesized using laser melting deposition. A dislocation density-based model was selected to analyze the mechanisms of the improvement in the wear resistance of the friction interface layer from initial wear stage to stable wear stage. Results show that GNMCs exhibit excellent antifriction and wear resistance in the stable wear stage due to the formation of friction interface layer with nanocrystalline structure. Grain boundary strengthening is the predominant strengthening mechanism of friction interface layer of GNMCs. Dislocations are blocked at grain boundaries to form complex dislocation barriers, further contributing to the high strength and wear resistance. The dislocation density-based model can be successfully used to predict the mechanical response of the friction interface layer subjected to the grain refinement during sliding friction process.

Microstructure and High-Temperature Wear Performance of FeCr Matrix Self-Lubricating Composites from Room Temperature to 800 °C.

FeCr matrix high-temperature self-lubricating composites reinforced by Mo, Ag, and CuO were fabricated by the powder metallurgy technique. The tribological behaviors of composites were studied at temperatures up to 800 °C. The CuO content was optimized according to the tribological results. Mo showed an obvious lubricating effect when it converted into MoO3. The bimetallic oxide system formed high-temperature solid lubricants with low shear strength. CuO reacted with MoO3 and formed CuMoO4 and Cu3Mo2O9. The composites showed an increase in the friction coefficient with the increase of CuO. However, the wear rates decreased with the increase of CuO. The critical threshold at which there was a transition of friction coefficients and wear rates from room temperature (RT) to 800 °C was 10 wt.% CuO. The Fe(Cr)-14% Mo-10.5% Ag-10% CuO composite showed the most reasonable high-temperature tribological behaviors. This was ascribed to the synergistic effects of silver, Mo, in situ formed solid lubricants (metal oxides and salt compounds), and the stable oxide film on the worn surfaces. At elevated temperatures, the dominant wear mechanism was oxidation wear.

Friction of metal-matrix self-lubricating composites: Relationships among lubricant content, lubricating film coverage, and friction coefficient

Metal-matrix self-lubricating composites can exhibit excellent tribological properties owing to the release of solid lubricant from the matrix and the formation of a lubricating film on the tribosurface. The coverage of the lubricating film on a worn surface significantly influences the sliding process. However, it is difficult to quantify the film coverage owing to the thin and discontinuous character of the lubricating film and the high roughness of the worn surface. A quantitative characterization of the lubricating film coverage based on X-ray photoelectron spectroscopy (XPS) analysis was developed in this study. The friction tests of Cu-MoS2 composites with a MoS2 content of 0–40 vol% were conducted, and the worn surfaces of the composites were observed and analyzed. Further, the influence of the MoS2 volume content on the coverage of the lubricating film on the worn surface was investigated. The relationships among the volume fraction of the lubricant, coverage of the lubricating film, and the friction coefficient were established. The friction model for the metal matrix self-lubricating composites was developed and verified to facilitate the composition design and friction coefficient prediction of self-lubricating composites.

Tribological performance of Ni3Al matrix self-lubricating composites containing Ag prepared by laser melting deposition

For improving tribological properties of Ni3Al-based alloy, Ni3Al matrix self-lubricating composites with 10 wt% Ag are successfully fabricated by spark plasma sintering (SPS) and laser melting deposition (LMD), which are abbreviated as NAS and NAL, respectively. Tribological behavior of NAS and NAL against Si3N4 ball under constant load (12 N) and variable speeds (from 0.2 to 1.1 m s−1, 0.3 m s−1 in tolerance) are systematically investigated. The results showed that NAL exhibited lower friction coefficients (0.28–0.36) and considerable wear resistance (4.6–5.9 × 10−5 mm3 N−1 m−1) as compared to NAS. It should be noticed that the tribological performance of NAL is excellent at 1.1 m s−1, while the tribological behavior of NAS is deteriorated significantly. LMD, as a process of layer by layer deposition, results in that each layer is comprised of the freshly deposited materials and the previously solidified materials. The characteristic of LMD is particularly significant for obtaining the stable bonding between layer, leading to the improvement of good interface bonding capability between tribolayer and substrate during the sliding wear process. Furthermore, the stability of the tribolayer is favorable for the reduction of friction coefficient and wear rate. For NAS, the local exfoliation of lubricant layer and the occurrence of naked substrate at the sliding speed of 1.1 m s−1 is explainable for the poor tribological performance.

Effects of Deep Cryogenic Treatment on the Microstructures and Tribological Properties of Iron Matrix Self-Lubricating Composites

The effects of deep cryogenic treatment on the microstructures and tribological properties of the self-lubricating iron matrix composites are investigated. The self-lubricating composites are deeply cryogenically treated at about −196 °C. The results show that with deep cryogenic treatment, the martensite phase transformation occurred from phase γ to α′, and the fine particle carbides precipitated between martensites with the extension of cryogenic treatment time, measured by X-ray diffractometry (XRD) and scanning electron microscope (SEM). Compared with the as-sintered specimen, the maximum hardness of the specimens processed by cryogenic treatment increases by 172.8% from 253.2 HV to 690.7 HV. The materials with deep cryogenic treatment for 8 h show the best tribological properties, i.e., the average friction coefficient decreases by 75% from 0.36 to 0.09, and the wear coefficient decreases by 63% from 341 to 126 × 10−6 mm3/Nm at 150 N and 8 mm/s. The improvement of the tribological property can be primarily attributed to the martensite phase transformation from γ to α′ and the precipitation of fine particles carbides between the martensites, which increase the hardness and the wear resistance after the cryogenic treatment.

Differences in tribological performance between spark plasma sintering and laser melting deposition for fabrication of Ni3Al matrix self-lubricating composites

Ni3Al matrix self-lubricating composites containing multilayer graphene and Ag are successfully prepared by spark plasma sintering (SPS) and laser melting deposition (LMD), which are denoted as NMAS and NMAL, respectively. The sliding wear performances of NMAS and NMAL at 25–500 °C are systematically investigated. NMAL exhibits the lower friction coefficients (0.20–0.40) and considerable wear resistance (2.8–5.0 × 10−5 mm3 · N−1 · m−1) as compared to NMAS. For the use of spherical prealloyed powders and the manner of layer by layer deposition, the hardness distribution of NMAL is more uniform than that of NMAS. NMAL shows the better tribological properties in comparison to NMAS from 25 to 400 °C because of the good consistency of microhardness. NMAL has excellent tribological behavior at 400–500 °C, while NMAS exhibits poor tribological performance. An explanation for the result is that the lubricating film of NMAL is still intact at 400–500 °C, while the lubricating film of NMAS is cracked and peeled off.

Effect of sintering temperature on tribological properties of Ni60 matrix self-lubricating composites

Ni60-based self-lubricating composites were prepared by vacuum sintering, and the tribological properties were evaluated against commercial Si3N4 ball at room temperature. Apparently, the experimental results show that the prepared composites exhibit excellent lubricating properties at room temperature. The average friction coefficient of the samples is between 0.13 and 0.35 at different sintering temperatures. It can be found that as the sintering temperature increases, the wear rate decreases. Its hardness value showed an upward trend between 900°C and 1000°C, while the temperature achieves 1000°C, the hardness of the composite decreases, because the lubricating phases such as Sr2MoO4 and Sr (CrO4) were produced to increase the porosity.