Self-lubricating Ceramic Composites Research Articles

High-pressure synthesis and performance analysis of WC-cBN-MoS2 self-lubricating ceramic composites

WC-cBN-MoS2 self-lubricating ceramics were synthesized by using high pressure and high temperature sintering method. An analytical model of the self-lubricating properties has been developed to reveal the interrelationship between mechanical and self-lubricating properties. The influence of mechanical and self-lubricating properties of ceramics on the wear rate were analyzed. Self-lubricating performance optimization criterion are proposed accordingly. It was found that MoC, WS2 can be formed during sintering, which can ensure self-lubricating and low temperature sintering properties. Adhesive wear is the main wear mechanism, the extent of which varies with the content of self-lubricating components. The high modulus of elasticity is the main reason for the excellent self-lubricating and mechanical properties of composites. The well-sintered WC-cBN-MoS2 bulks present an excellent self-lubricating property with a hardness of 26.3 GPa and fracture toughness of 10.5 MPa·m1/2, which is improved at least by 52% and 46% respectively compared with the traditional self-lubricating ceramics.

Enhancement of Dry Sliding Tribological Characteristics of Perforated Zirconia Toughened Alumina Ceramic Composite Filled With Nano MoS2 in High Vacuum

Abstract An innovative approach was adopted for fabricating zirconia toughened alumina (ZTA)-MoS2 self-lubricating ceramic composites with the inclusion of hydrothermally synthesized nano MoS2 through the micropores of perforated ZTA ceramics. This method avoided the exposure of MoS2 in high-temperature environment due to its in-sensitiveness during traditional sintering techniques. Different weight percentages (wt%: 0, 5, 10, and 15) of graphite were incorporated to produce porous ZTA ceramics with the help of cold press sintering followed by insertion of nano MoS2 into the matrices. Best tribological characteristics were obtained with 10 wt% graphite-ZTA-MoS2 specimen which offered an improvement of ∼66% in coefficient of friction (COF) and ∼96% in specific wear rate when dry sliding tests were carried out against silicon nitride (Si3N4) in high vacuum (5.0 × 10−4 mbar). Nano MoS2 was sheared off at the contacting interface during sliding under load which showed a good tribological characteristics of the composite. Delamination was found as the dominating wear mechanism in ZTA-MoS2 composites during wear tests.

Synthesis of CaF2 Nanoparticles Coated by SiO2 for Improved Al2O3/TiC Self-Lubricating Ceramic Composites.

In order to reduce the influence of CaF2 addition on the mechanical properties of self-lubricating ceramic tools, we applied a silicon dioxide (SiO2) coating on calcium fluoride (CaF2) nanoparticles through hydrolysis and condensation reactions using the tetraethoxysilane (TEOS) method. The powder was dried by the azeotropic method, so that it acquired a better dispersibility. The resulting composite powders were characterized using XRD (X-ray diffraction) and TEM (transmission electron microscopy), showing that the surface of CaF2 was coated with a layer of uniform and compact SiO2. SiO2 shells with different thicknesses could be obtained by changing the amount of TEOS added, and the thickness of the SiO2 shells could be controlled between 1.5 and 15 nm. At the same time, a ceramic material containing CaF2 nanoparticles and CaF2@SiO2-coated nanoparticles was prepared. It had the best mechanical properties when CaF2@SiO2-coated nanoparticles were added; its flexural strength, fracture toughness, and hardness were 562 ± 28 MPa, 5.51 ± 0.26 MPa·m1/2, and 15.26 ± 0.16 GPa, respectively. Compared with the ceramic tool containing CaF2 nanoparticles, these mechanical properties were increased by 17.57%, 12.67%, and 4.88%, respectively. The addition of CaF2@SiO2-coated nanoparticles greatly improved the antifriction and wear resistance of the ceramic material, and the antifriction and wear resistance were balanced.

An advanced self-lubricating ceramic composite with the addition of core-shell structured h-BN@Ni powders

Aiming at the contradiction between tribological and mechanical properties of traditional self-lubricating ceramic composites, an advanced self-lubricating ceramic composite with the addition of pure-metal coated solid lubricant powders was developed in terms of microstructural modification of the traditional composites. Core-shell structured h-BN@Ni powders were synthesized by electroless plating technique with hydrazine hydrate as reducing agent. Al2O3/(W,Ti)C/h-BN@Ni composite made by adding h-BN@Ni powders exhibited notable improvements in microstructure and mechanical properties as compared with Al2O3/(W,Ti)C/h-BN composite made by adding uncoated h-BN powders. EDS and TEM analyses showed that the Ni shell particles in the Al2O3/(W,Ti)C/h-BN@Ni composite concentrated around the h-BN core grains and were located at grain boundaries or within the matrix grains. Due to their flaky structure, the h-BN grains had orientation distribution in the as-prepared ceramic composites, which resulted in anisotropies in mechanical properties and tribological behaviors. Dry sliding friction test revealed that the Al2O3/(W,Ti)C/h-BN@Ni composite had lower friction coefficient and higher wear resistance than the Al2O3/(W,Ti)C/h-BN composite. The different h-BN release modes caused tribological behaviors of the friction surface perpendicular to the hot pressing axis (HPA) to be better than those of the friction surface parallel to HPA.

Structure design of Al2O3/TiC/CaF2 multicomponent gradient self-lubricating ceramic composite and its tribological behaviors

To overcome the conflict between the tribological and mechanical properties of traditional self-lubricating ceramic composites, an Al2O3/TiC/CaF2 multicomponent gradient self-lubricating ceramic composite was structurally designed by using a compositional gradient model for multicomponent functionally gradient materials. According to the model, the Al2O3/TiC/CaF2 gradient composite was gradient-controlled using two gradient indexes n1 and n2, and was accordingly categorized into four types with different gradient systems. A proper gradient system was selected by calculating the distribution of residual thermal stresses via an analytical method and the finite element method. The as-prepared Al2O3/TiC/CaF2 gradient composite was characterized by reducing the distribution of CaF2 solid lubricants from its surface layers to its middle layer and the residual compressive stresses existing in the surface layers. The measurement obtained using the indentation crack method showed that both the surface layers and middle layer were subjected to residual compressive stresses. Compared with the homogeneous composite, the flexural strength, Vickers hardness, and fracture toughness of the gradient composite increased by 21%, 16%, and 5.9%, respectively. The results of the dry sliding friction test demonstrated that the gradient composite had comparable antifriction property and superior wear resistance to the homogeneous composite.

Novel Approach to the Fabrication of an Alumina-MoS2 Self-Lubricating Composite via the In Situ Synthesis of Nanosized MoS2.

The fabrication of a self-lubricating ceramic composite with MoS2 as the solid lubricant is extremely difficult given the high temperature sensitivity of MoS2. In this study, a hydrothermal method was utilized for the in situ synthesis of nanosized-MoS2 in Al2O3 ceramic to fabricate an Al2O3-MoS2 self-lubricating composite. The composite exhibited excellent self-lubricating properties with low friction coefficient and wear rate in a high-vacuum environment because of the efficient formation of self-lubricating films from the finely structured MoS2 in the Al2O3 matrix. The results of this study establish a new approach to the fabrication of self-lubricating ceramic composites with temperature-sensitive solid lubricants.

Research on Friction and Wear Performance of TiB2Based Self-lubricating Ceramic composites

TiB 2 based self-lubricating ceramic composites with the addition of different content of h-BN were prepared by hot pressing technique and their mechanical properties were tested. Friction and wear experiments are conducted on a MMW – 1A configuration control universal friction and wear testing machine. Friction and wear properties of the ceramic composites under different speed and load were studied. The results show that with the increase of the content of h-BN, the hardness, flexural strength and fracture toughness showed a trend of decrease. When load was fixed, with the increase of rotational speed, the friction coefficient rose and the wear rate were first decreased and then increased. When rotational speed was fixed, the friction coefficient and wear rate first decreased and then increased with the increase of load. In nitrogen, the friction coefficient and wear rate of material were higher than those in the air.

Synthesis of (h-BN)/SiO2 core–shell powder for improved self-lubricating ceramic composites

An easy and effective preparation method has been achieved for the fabrication of core-shell-structure (h-BN)/SiO2 composite powders. Ammonium hydroxide (NH4OH) was used as a catalyst. Silica (SiO2, ~50–100nm) was coated on the surface of lamellar hexagonal boron nitride (h-BN, ~5–10μm) particles via hydrolysis and condensation reaction of tetraethylorthosilicate (TEOS). The resulting composite powders were characterized using XRD (X-ray diffraction), SEM (Scanning electron microscopy), EDS (Energy-dispersive X-ray spectroscopy) and TEM (Transmission electron microscopy). A completely compact and uniform SiO2 coating binds tightly to h-BN particles. In addition, the effects of the reaction temperature and molar ratio of H2O and NH4OH between TEOS on the coating results are studied. The coating processes of the composite powders reveal that SiO2 nucleates and grows first on the surface of h-BN particles. By adding (h-BN)/SiO2 instead of h-BN as a solid-lubricant to Al2O3/TiC, the resulting microstructure, fracture-toughness and flexural strength of the composite are noticeably improved.

Friction and wear properties of (WAl)C–Co ceramic composites under sea water environment

In this paper, (WAl)C–Co/fluoride(CaF2, BaF2, CaF2/BaF2) self-lubricating ceramic composites were prepared by mechanical alloying and hot pressing sintering. The tribological behavior of the composite under sea water environment was evaluated. [(W0.67Al0.33)C0.67–10wt% Co]/10wt% CaF2 composite show a coefficient of friction of 0.15 and a wear rate of 1.3×10−5mm3/(Nm) under 60N at 0.067m/s. Sea water play a key role on improve the tribological behavior of the composites through form Al2O3 and H2SiO3 (SiOx after wear) on the worn surface.

Investigation of (WAl)C–Co ceramic composites with the additions of fluoride solid lubricants: Preparation, mechanical properties and tribological behaviors

(WAl)C–Co/fluoride (CaF2, BaF2, CaF2/BaF2) self-lubricating ceramic composites prepared by mechanical alloying and hot pressing sintering was reported for the first time. The influence of the fluorides on the mechanical properties, friction coefficient and wear rate of the composites was also evaluated. The friction coefficient of the (W0.67Al0.33)C0.67–Co/fluoride materials is in the range of 0.22–0.36, with the wear rate in the order of 10−6mm3N−1m−1. The fluoride in the (W0.67Al0.33)C0.67–Co/fluoride plays a key role in decreasing the friction coefficient and wear rate. The wear layer formed during friction process exerts a significant influence on the tribological properties, which would increase the friction coefficient but protect the worn surface.

Effects of Different Solid Lubricants on Mechanical and Tribological Properties of Al<sub>2</sub>O<sub>3</sub>/ZrO<sub>2</sub> Nanocomposites

Solid lubricants were suited for the applications of high temperature parts. Recently, researches for self-lubricating ceramic matrix composites using solid lubricants, such as MoS2, WS2, h-BN, and graphite etc., became active. This work is to use the spark plasma sintering (SPS) technique in order to make self-lubricating Al2O3/15wt% ZrO2 nanocomposites with higher mechanical properties. To optimize the self-lubricating Al2O3/15wt% ZrO2 nanocomposite systems were added with different solid lubricants (CaF2, BaF2, MoS2, WS2, h-BN, and graphite). The coefficient of friction of CaF2, h-BN and graphite added composites in steady-state at room temperature are below 0.3.

A mechanical model for surface layer formation on self-lubricating ceramic composites

To predict the thickness of a self-lubricating layer on the contact surface of ceramic composite material containing a soft phase during dry sliding test, a mechanical model was built to calculate the material transfer of the soft second phase in the composite to the surface. The tribological test, as a case, the contact situation of a composite disk sliding against a loaded pin was studied in the model. The macroscopic deformation of the composite was supposed to be elastic. Subsurface stresses were calculated by the Hamilton's formulae. Further, the squeezed second phase material of inclusions was analyzed microscopically based on the Hashin's equations of inelastic inclusions. Based on the mechanical model, the thickness of the transferred layer during the multi sliding passes was predicted. The influence of material parameters and loading conditions such as inclusion (second phase) concentration in the composite, yield strength of the inclusion material, normal load on the pin and friction force were studied. The model can be used to design and optimize self-lubricating ceramic composites for applications and resulting contact conditions.

Tribological properties of self-lubricating CMC/Al 2 O 3 pairs at high temperature in air

Three different self-lubricating ceramic matrix composites (CMCs) were fabricated by hot-pressed sintering. They are: Al2O3-50CaF2, Al2O3-20Ag20CaF2, and Al2O3-10Ag20CaF2. Tribological tests were performed at temperatures ranging from 20°C to 800°C in air using a pin-on-disk tester. The experimental results show that the addition of the solid lubricants CaF2 and Ag can evidently reduce the friction coefficients of alumina between 200°C and 650°C but not at room temperature and the wear rate of disks and pins at elevated temperature. The improvements in the friction and wear properties of CMC were due to the formation of a well-covered solid lubricating film. However, breakdown of the lubricating films at 800°C resulted in high friction and wear.