Wear Behaviour

The wear behaviour of engineering ceramics is relatively complex and is subject to many variables.

Cracking, plastic deformation, tribochemical interaction, abrasion and surface fatigue have all been identified as wear mechanisms operative in ceramic sliding wear situations.

The individual ceramic microstructures also affect the wear behaviour in a fundamental manner.

Wear Mechanisms

When one considers the intimate contact of two sliding surfaces where hard particles are either present or formed during sliding, abrasive wear can occur as a consequence of both plastic deformation and fracture mechanisms.

However, in polycrystalline ceramics, the amount of plastic deformation that can occur is strictly limited by the available slip systems and twinning modes. Consequently, abrasive wear is aided by fracture mechanisms initiated by the inelastic structure of the material (figure 1 below).

Also, on a more microscopic scale than the cracking shown in figure 3.1, the intersection of slip bands or twins with barriers such as grain boundaries, particles or other slip bands, can commonly lead to stresses which often give rise to crack nucleation and growth.

Although plastic deformation and fracture have been observed to result in material removal during the abrasive wear of brittle solids, the predominant and rate controlling mechanism differs for both different wear environments and different materials.

Wear Mechanism	Contributory Factors
Microfracture (Trans and Intergranular) Surface Cracking	Stress Concentration Second Phases Flaws High Young's Modulus Low Fracture Toughness

Microfracture (Trans and Intergranular) Subsurface Cracking

Residual Stresses Inclusions Flaws Second Phases Low Fracture Toughness

Delamination Fracture (Subsurface Cracking Due to Fatigue)

Plastic deformed layer Tribochemical reaction layer Residual stresses Inclusions, second phases Flaws

Tribochemical Reaction

Surface Layers Stress Corrosion (Environment) Surfaces Effects (Rehbinder) Oxidation Sliding Velocity

Microfracture (Trans and Intergranular) Surface Cracking

Surface Softening Plastic Deformation Structural Changes (crystal structure) Thermal Shock Cracking Sliding Velocity

Microfracture (Trans and Intergranular) Surface Cracking

Transferred Material (adhesion, roughness) Loose Wear Debris Compacted Wear Debris

Microfracture (Trans and Intergranular) Surface Cracking

Microcutting, Microploughing Microfatigue Microcracking Spalling

Wear Mechanisms in Ceramics (After Bundschuh and Zum Gahr, "Influence of porosity on Friction and Sliding Wear of Tetragonal Zirconia Polycrystal", Wear, 151, (1991), 175.)

Figure 1: Crack Modes in Polycrystalline Ceramics

Plastic deformation is favoured when the load on the abrasive particles is small. This occurs as a result of small abrasive particles or low applied loads, when the abrasive is blunt or blunts during contact, and when the ratio of fracture toughness to hardness is high.

Conversely, indentation fracture is favoured when the load on the abrasive particles is high. As occurs with large particles or high applied loads, when the abrasive is sharp or remains sharp due to fracture on contact, and when the ratio of fracture toughness to hardness is low.

This ratio of fracture toughness to hardness has also been shown to be of significance in the erosive wear behaviour of zirconia ceramics. The high value for this ratio with Technox Zirconia ceramics leads to their excellent erosive wear resistance in applications such as pumps and choke valves.

Please contact our sales engineers who can advise on the optimum material selection for wear resistance in your application and environment.