Ceramic’s exceptional properties in the areas of mechanical strength and extreme hardness make these a versatfie class of materials. In combination with other properties such as dimensional stability and temperature and corrosion -resistance, both traditional and “advanced” ceramics can be used in a wide range of wear resistant applications. Alumina, silicon carbide, silicon nitride, and others are at your disposal to resist the most severe wear and abrasive conditions.
Wear behavior of ceramics is determined by the bond forces developing between the two materials, whether the other material is another ceramic or a metal. In the atomically clean state, ceramics form strong bonds of adhesion, like metals. These bonds are influenced by a variety of bulk and surface properties.
Surface properties include electronic surface states, ionic species present at the surface, chemistry of the contrasting material, and the nature of surface contaminants present. Bulk properties include crystallography, cohesive bonding energy, and the presence or absence of defects.
Metals generally deform plastically compared to most ceramics that are brittle and fracture with little or no plastic flow. However at the interface between two ceramics in solid-state contact under load and relative motion, plastic flow has been observed in the surface layer of some ceramics.
But unlike metals, gross fracture and plastic deformation can occur when the elastic limit is exceeded. With tangential motion, the forces required for brittle fracture are less than required with normal loading.
Influencing factors that affect plastic flow include crystal stricture, dislocation vacancies and stacking friction. All these phenomena consequently affect the adhesion, friction and wear behavior of ceramics. In addition the presence of surface films, such as absorbates, influence the behavior of both metals and ceramics.
With ceramics and other ionic solids, the presence of films such as water and surface-active organics can affect adhesion, friction, and wear by altering the amount of plastic deformation during sliding or rubbing. Partially-stabilized zirconia has been shown to have increased wear in aqueous environments, with an increase in the coefficient of friction. Certain lubricants, such as fatty acids, can reduce the coefficient of friction though others may have the opposite effect.
For oxide ceramics, such as alumina, used in tribological applications, adhesion behavior is significantly affected by both structure and surface chemistry. Adhesion and friction characteristics are anisotropic, similar to that observed with hexagonal metals. In other words, the lowest coefficient of friction is observed when sliding on the preferred slip plane in the preferred slip direction on that plane,
When ceramics come into contact with metals, the large differences in elastic and plastic deformation can result in plowing of the softer material, Therefore, an increase in wear of one or both of the materials can occur. Furthermore, adhesion of the metal, if it forms stable oxides, can occur to the oxygen ions in the outennost atomic layer of the ceramic. However, strong chemical bonding does not
occur at the interface with metals that do not form stable oxides. Adhesion is weak and wear to the ceramic is absent. The strength of the bond is related to molecular-orbital energies and whether antibonding or bonding orbitals are occupied.
With ferrite ceramics, adsorbates are present on the surface from the environment. The removal of such films produces very strong intcrfacial adhesion and high friction. The ferrites act similarly to oxide ceramics when in contact with metal materials.
The metal-ferrite adhesive bond at the interface is a chemical bond between the metal atoms and the large oxygen ions in the ferrite surface. The strength of this bond is related to oxygen-to-metal bond strength in the metal oxide. The less active the metal, the less adhesion and transfer there is to the ferrite. With transition metals, greater percentages of d-bond character reduce metal activity and lower adhesion and friction. In addition, the environment can play an important role in wear behavior of ferrites. Adsorbed nitrogen reduces adhesion and friction; oxygen, on the other hand, increases adhesion and friction. The latter holds true for ferrite-to-metal interfaces.
Nonoxide ceramics, such as silicon carbide, are similar to oxides in wear behavior. Friction and wear also are anisotropic under both adhesive and abrasive conditions. Adsorbates on the surface again play a significant role, By increasing the temperature, the adsorbates are removed producing an increase in friction. At even higher temperatures, friction can decrease rapidly due to graphitization of the surface for carbide materials. A lubricating film is thus produced from the material itself.
Silicon carbide and other nonoxides act similarly to oxides when in contact with metals. Less-active metals produce lower friction. With transition metals, the weakest bond in the interfacial region is the metal bond. Therefore, the shear strength of the metals correlates with the coefficient of friction, Higher shear strengths produce lower friction. Metals that have low strength and in low percent dbond character transfer more to the ceramic material than with those having higher values.
The three basic factors affecting wear plastic deformation, cracking, and chemical interaction-also are influenced by such microstructural characteristics as pores and grains. Amorphous materials, such as glasses, undergo plastic deformation by viscous flow- this deformation is isotropic and occurs only at high local stresses, During the forming of glass the presence of both deformed and undeformed material causes large srre.cse.s that lead to cracking. This cracking is a major source of wear in amorphous materials.
Cracking in crystalline ceramics, on the other hand, can be caused by the deformation forces of slip, twinning, or both, However, as already mentioned, deformation is anisotropic. Therefore, lower than normal stresses can cause deformation that can increase surface roughness and surface friction. Consequently, plowing can occur which leads to more plastic deformation and cracking.
Grain boundaries can act as barriers to slip bonds or twins. The resultant stresses can lead to crack nucleation and growth, which influence wear. As with metals, ceramics show a grain-size dependence of deformation-in other words, the ease of plastic deformation varies inversely with the square root of grain size. in general, as grain size increases, cracking from stresses should increase above some minimum threshold. Wear increases with increasing grain size at a rate greater than expected just from plastic deformation.
Matters become even more complicated if grain size varies. Some grains may chemically interact more than others, leading to a mismatch in both elastic and thermal stresses. These stresses can cause fracture along and across grain boundaries as well as across the grains themselves, As cracking usually is initiated above a critical grain size, wear increases with increasing grain size. Porosity also has a negative effect on wear. The effect is greater in compressive loading than in tensile loading. With tensile loading, poreinitiated cracks propagate immediately to catastrophic failure. However, compressive loads do not cause immediate failure, allowing time for more extensive crack development leading to more wear. In general, wear increases exponentially with porosity.
If pores act as sources for slip bonds or twins, crack nucleation and growth increase and, consequently, so does wear. However, if slip bonds or twins terminate on pores, the potential tor cracks is reduced with a corresponding reduction in wear. The location of pores is another important factor. As cracking usually occurs at the grain boundaries, pores within the grains have little or no effect. As grain boundary pores also differ in shape, being lenticular or triangular rather than spherical, stresses are higher and lead to enhanced cracking along grain boundaries.
Of course pore size is another critical factor. Pores larger than the grains have a significant effect on cracking. If such large pores are near the surface, they can lead to punching, gouging, and plowing of the surface. Furthermore, large pores can cream larger elastic and thermal mismatches, causing an increase in localized stresses.
The crystalline structure of ceramics, along with the number and type of phases, has a marked influence on wear. Single-phase ceramic materials show a substantial increase in wear with increasing grain size. The inverse of wear follows a Hall-Fetch relation with the grain size dependence more pronounced in noncubic material. The effect of these crystalline anisotropies is to increase the intergranular fracture with a corresponding increase in grain size.
Transformation-toughened ceramics, such as partially-stabilized zirconia, have received attention for various wear applications. However, the amount of the Y0 stabilizer present controls the hardness. Below 5 wt% monoclinic Z0 content increases, which can lead to microcracking. Consequently, PSZ may not be suitable for wear applications where high local loads are present and may be better suited for more distributed load conditions.
Similarly, the wear properties of silicon carbide may be affected by the amount and distribution of such second phases as Si. Wear is lower in Si-containing materials, though wear increases with grain size, with a Hall-Fetch relationship. The higher wear and greater grain-size dependence of Si-C without free Si is attributed to elastic and thermal expansion anisotropics. Compared to MgO-stabilized PSZ, the Y0-PSZ has the lowest wear, though both materials’ wear behavior is affected by chemical interactions. For instance, a thick transfer film may form, out of another oxide such as iron oxide, which provides protection against wear. High surface stresses can cause continual particle formation which increases wear. The materials that had higher toughness can withstand higher normal loads without chipping and cracking compared to lower toughness zirconias. In general, the equation W = kPVt (where k = constant, W = wear rate, P = normal load, V = sliding speed, and t = sliding time; a,b,c, varies according to material) adequately describes the wear of toughened ceramics.
Such Zr0 materials, with their low coefficient and high KI have greater resistance than sintered SiC or reaction-bonded SiN , to surface damage induced by contact stress. The SiC friction behavior is dominated at high temperature by a viscous surface layer, which lowers the resistance to surface damage and increases the coefficient of friction. On the other hand, Zr0 materials maintain their friction behavior at higher temperatures-up to 1740°F (950°C)-and have been used successfully for powder-metal extrusion dies.
The higher toughness of MgO-PSZ materials is attributed to the tetragonal precipitates that nucleate and grow homogeneously within the cubic grains. These precipitates reduce stresses around a propagating crack and transform to the monoclinic form with accompanying expansion and shear. The expansion produces compressive stress that neutralizes the local tensile stress and stops the crack.
Several hardness tests are available to measure the resistance to wear and abrasion. Scratch tests are relatively simple but are inaccurate and inconsistent. One grinding test measures the rate of removal of material from a unit area of glass using loose abrasive. Each abrasive particle produces high local pressure to develop a crack. Because glass has a smooth surface, little material is removed at first but as the cracking increases, the rate of removal increases with time until the original surface is removed.
Impact abrasion hardness tests are based on depth of penetration produced by a sandblast of standardized generating condition as well as abrasive grains. Increasing the hardness of the abrasive grain in these tests reduces the hardness ratio between different glasses.
Indentation hardness is the standard for metallurgy and also can be used for glasses, ceramics, and glass-ceramics. Under suitable conditions, the point of a diamond pyramid is applied to the surface and to leave a permanent indentation. A combination of plastic deformation, plastic flow, and compaction beneath the indentation form the indentation. However, glass reacts differently to indentors of different shapes so the hardness values are not comparable. Two indentors are commonly used-the Knoop indentor and the diamond pyramid. The value of displaced material for the latter has been about 60% greater than for the Knoop indentation for most glasses, causing discrepancies between the two different tests. In general, indentation measurements correlated with other hardness measurements, the elastic modulus, and the softening temperature.
For applications where glass or glass-ceramic parts are sliding or rotating with each other, the coefficient of friction must be measured. Though glass surfaces are usually very smooth, foreign substances are quickly adsorbed, lowering the static coefficient to values similar to those found between metals. As with ceramics, lubricating coatings, such as oils can reduce the static coefficient significantly. Similarly, fatty acids such as stearic, also can reduce the coefficient. In addition, water-dispersed liquids containing silicones reduce the coefficient to very low values. Such lubricating materials form a resistant film that protects the surface from abrasion.
|Tensile strength psi||18K||20K||18.5K||25K-|
|Flexural strength psi||42.5K||46K||46.5K||45K-|
|Modulus of elasticity psi x 106||32||39||42||43-46||45||50||55||56||8||38|
|Impact resistance in./lbs charpy||6.3-6.5||6.5||6.5-6.8||6.5-6.7||7||7||6||7||3.3|
|Tensile strength psi|
|Flexural strength psi||100K||5K||10K||37K||80K||47K||8K||30K||125K|
|Modulus of elasticity psi x 106||30||2||17||56||59||55||30||24||45|
|Impact resistance in./lbs charpy|