Hard Ceramic Phases Research Articles

Phase composition and tribomechanical properties of Ti–B–C nanocomposite coatings prepared by magnetron sputtering

Protective nanocomposite coatings based on hard ceramic phases (TiC, TiB2) combined with amorphous carbon (a-C) are of interest because of their adequate balance between mechanical and tribological performances. In this work, Ti–B–C nanocomposite coatings were prepared by co-sputtering of graphite and TiB2 targets. Varying the discharge power ratio applied to the graphite and TiB2 targets from 0 to 2, the a-C content in the coatings could be tuned from 0 to 60%, as observed by means of Raman and x-ray photoelectron spectroscopy (XPS). The microstructural characterization demonstrated a progressive decrease in crystallinity from an initial nanocrystalline (nc) TiB2-like structure to a distorted TiBxCy ternary compound with increasing C concentration. X-ray absorption near-edge structure measurements on the B K-edge helped to determine a hexagonal arrangement around the B atoms in the ternary TiBxCy phase. A fitting analysis of the C 1s XPS peak allowed us to evaluate the relative amount of a-C and TiBxCy components. A drastic change in hardness (from 52 to 13 GPa) and friction coefficient values (from 0.8 to 0.2) is noticed when moving from nc-TiB2 to TiBC/a-C nanocomposites. The fraction of a-C necessary to decrease the friction below 0.2 was found to be 45%. Raman observation of the wear tracks determined the presence of disordered sp2-bonded carbon phase associated with the diminution of the friction level.

Recubrimientos de materiales compuestos metal-cerámico obtenidos por nuevas tecnologías de proyección térmica: Proyección fría (CGS) y su resistencia al desgaste

In this paper, composite coatings composed by an aluminum bronze metal matrix and a hard ceramic alumina phase obtained by cold spray technique were obtained in order to increase the tribological properties of the pure bronze coatings. The different processes that occur during the coating formation (hardening of the metal particles, fragmentation of the ceramic particles, shot peening on the metal substrate, etc) are described and their effects on the coating properties are studied. Wear tests consisting on Ball-on-Disk tests, abrasion Rubber Wheel tests and erosion tests as well as microhardness and adhesion tests are carried out and the results are correlated with the ceramic phase content of the coatings. It can be concluded that the hard ceramic phase increases the tribological properties with relation of the initial bronze coating. Finally, main wear mechanisms during the tribological tests are described. Keywords

Microstructure and mechanical properties of TiB2–TiC–Al2O3–SiC composite coatings developed by combined SHS, sol–gel and laser technology

A ceramic matrix composite (CMC) coating with a novel composition (TiB2–TiC–Al2O3–SiC) is developed by combining SHS (self-propagating high temperature synthesis), Sol–gel and laser technology. TiO2 and SiO2, were obtained as nano-particulate powders from Sol–gel technology. Multiple SHS reactions are laser-triggered in a mixture of aluminum (Al), boron carbide (B4C) and graphite (C) powders and sol–gel derived nano-particulate titanium oxide (TiO2) and silicon oxide (SiO2) powders, to yield hard ceramic phases that are subsequently laser clad onto substrates, namely AISI 1020 steel and AISI 304 steel. Varying amounts of (SiO2 + C) are added with a fixed ratio of TiO2, Al and B4C to observe the effect of additional SiC on the properties of TiB2–TiC–Al2O3–SiC composite. The developed coatings exhibit appreciable increase in hardness, wear resistance and coefficient of friction (COF) with increase in proportion of (SiO2 + C) in the precursor powder mixture.

Microstructural analysis and properties of Al–Cu–Mg/bagasse ash particulate composites

The effects of bagasse ash particles on the as-cast microstructure and properties of Al–Cu–Mg alloy composites produced by double stir-casting method have been studied. 2–10 wt% bagasse ash particles were added. The microstructure and phases of the alloy particulate composites produced were examined by SEM/EDS and XRD methods. The physical and mechanical properties determined includes: density, ultimate tensile strength, yield strength, compression strength, hardness values and impact energy. The results revealed that, additions of bagasse ash reinforcement increased the hardness and compression strength values by 43.3% and 57.7%, respectively, the density and impact energy decreased by 10.27% and 45%, respectively, as the weight percent of bagasse ash increases in the composites. The yield strength and ultimate tensile strength increased by 49.76% and 34.25% up to a maximum of 8 wt% bagasse ash addition, respectively. These increases in strength and hardness values are attributed to the distribution of hard and brittle ceramic phases in the ductile metal matrix. The microstructure obtained reveals a dark ceramic and white metal phases, which resulted into increase in the dislocation density at the particles–matrix interfaces. These results show that better properties are achievable by the addition of bagasse ash to Al–Cu–Mg alloy.

Manufacturing, microstructure and wear resistance of novel hot extruded Fe based metal matrix composites

Materials with high resistance against abrasive wear are of high interest for many tool applications, e.g. in mining industry. These materials usually contain a large amount of hard ceramic phases embedded in a metallic matrix and can only be manufactured by powder metallurgy, most often via hot isostatic pressing. Here a new manufacturing route via powder presintering and direct hot extrusion is presented, which enabled manufacturing of Fe based metal matrix composites with up to 30% of fused tungsten carbide (FTC, WC/W2C) or titanium carbide (TiC). The matrixes chosen are hot and cold work tool steels. The manufacturing process is elucidated and microstructures formed are presented. The microstructure–property relation of the new Fe based MMC is discussed.

Effects of silicon carbide reinforcement on microstructure and properties of cast Al–Si–Fe/SiC particulate composites

The effects of silicon carbide (SiC) particles on the as-cast microstructure and properties of Al–Si–Fe alloy composites produced by double stir-casting method have been studied. A total of 5–25 wt% silicon carbide particles were added. The microstructure of the alloy particulate composites produced was examined, the physical and mechanical properties measured include: densities, porosity, ultimate tensile strength, yield strength, hardness values and impact energy. The results revealed that, addition of silicon carbide reinforcement, increased the hardness values and apparent porosity by 75 and 39%, respectively, and decreased the density and impact energy by 1.08 and 15%, respectively, as the weight percent of silicon carbide increases in the alloy. The yield strength and ultimate tensile strength increased by 26.25 and 25% up to a maximum of 20% silicon carbide addition, respectively. These increases in strength and hardness values are attributed to the distribution of hard and brittle ceramic phases in the ductile metal matrix. The microstructure obtained reveals a dark ceramic and white metal phases, which resulted into increase in the dislocation density at the particles–matrix interfaces. These results show that better properties is achievable by addition of silicon carbide to Al–Si–Fe alloy.

Laser melting of plasma-sprayed copper–ceramic coatings for improved erosion resistance

Copper-based coatings containing a hard ceramic phase can provide an engineering solution to components exposed to particle erosion environments provided the microstructure of these coatings is appropriate. Single-phase ductile copper is damaged by micromachining and ploughing while single-phase brittle materials are thought to be damaged by the generation and propagation of subsurface lateral cracks. Therefore, a multicomponent system composed of a ductile metal such as copper containing hard and brittle ceramic particles is expected to be tough enough to resist particle impacts at 90° and sufficiently hard to deflect eroding particles at low impact angles. Copper–ceramic coatings were manufactured by plasma spraying composite powders. These spray powders were obtained by agglomerating fine copper and boride powders by means of an organic binder in a rotating tumbler. The thickness of as-sprayed coatings reaches about 350 μm. After surface polishing, these coatings were laser melted with parameters ensuring in-depth melting. Polished as-sprayed, surface milled laser-melted coatings and bulk copper specimens were erosion tested at 25° and 300°C at impact angles of 25° and 90° in a laboratory erosion device. This device was designed to impact materials with coarse (32–300 μm) iron ore particles at a speed of 100 m/s. The evaluation of wear damages was done with a laser profilometer and scanning electron microscopy was used to evaluate microstructural changes upon laser treatment as well as surface modifications after erosion tests. Particle erosion resistance of laser-melted plasma-sprayed copper coatings containing 45 vol.% boride phase is improved in comparison with bulk copper. The low angle impact erosion is reduced at both 25°C and 300°C (1.7 times at 25°C and 4.8 times at 300°C) and the normal angle impact erosion resistance is the same as pure copper at both temperatures. As-plasma-sprayed coatings presented the worst erosion resistance. The improvement in erosion resistance of laser-melted coatings is attributed to the formation of large clusters of boride particles upon laser melting plasma-sprayed copper-boride coatings. These rounded boride islands are large enough to deflect the incoming eroding particles and the elastic copper phase dissipates the energy of particles with a high impingement angle.

Friction and wear behavior of [formula omitted] composites under dry sliding conditions

The wear resistance of MMCs can be improved by increasing the volume fraction of the reinforcing ceramic phase (HRC) by as much as 70%. Among the various types of HRC composites, a new Al 2O 3Al co-continuous ceramic/metal composite (referred to as C 4 composite) has been recently produced at The Ohio State University. A new method of preparation, based on a displacement reaction between a silica precursor (whose frame is faithfully reproduced in the composite product) and molten aluminum, allows the production of a MMC where both the Al 2O 3 and Al phases are continuous and interpenetrating. The aim of the present work was to investigate the tribological behavior of C 4 Al 2 O 3 Al composites under dry sliding conditions using a computer-controlled slider-on-cylinder tribometer. The tests were carried out at applied loads in the range 5–30 N and sliding speeds in the range 0.3–1.8 m/s. for sliding distances up to 10 km. Wear scars and debris were characterized by means of scanning electron microscopy (SEM), electron probe microanalysis (EPMA), with an EDS analyzer and X-ray diffraction (XRD) analysis. Under the adopted testing conditions, the C 4 composites underwent only mild wear, mainly as a consequence of the high load-bearing capacity of the ceramic phase which enhances wear resistance. The high wear resistance is also related to the nature of the third-body, mainly constituted by Fe 2O 3, which is produced by the abrasive action carried out by the hard ceramic phase on the counterfacing steel. Under the adopted testing conditions, the transition from mild to severe wear, observed at a critical load in conventional MMCs, was never observed in the C 4 materials. Both the continuity between aluminum and alumina, and the increase in interfacial bonding, played an important role in influencing the wear behavior of the C 4 composites. These aspects led to an improvement of wear resistance of the C 4 composites over conventional MMCs, under the same experimental conditions, and probably shifted the transition from mild to severe wear regimes to higher critical loads.

Laser alloying of a gradient metal-ceramic layer to enhance wear properties

The metallurgical structure, composition, hardness and abrasive wear resistance of a gradient layer were characterized following coating and melting of the layer by a CO 2 laser. The results show that WC particles, used as hard ceramic phases, incorporated in the layer varied gradually in volume fraction from the surface to the substrate. In the region where the coating material connected with the substrate, a sosloid with good toughness was obtained. The layer produced had a uniform structure and excellent bonding with the substrate and was free of pores and cracks. The thickness of the layer is in the range 0.4–0.8 mm. The superficial hardness is about 1100 HV/0.2. The structure of the gradient layer processed by this method differed from those of conventionally laser-melted layers. The microhardness of the gradient layer varied smoothly acccording to the volume variation of the hard ceramic phases. Thus, it can not only improve the wear resistance of the layer but also avoid the production of cracks.