where V is the volume worn away, W is the normal load, x is the sliding distance, H is the hardness of the surface being worn away, and k is a nondimensional wear coefficient dependent on the materials in contact and their exact degree of cleanliness. The term k is usually interpreted as the probability that a wear particle is formed at a given asperity encounter. Equation () suggests that the probability of a wear-particle formation increases with an increase in the real area of contact, Ar (Ar = W=H for plastic contacts), and the sliding distance. . | V kWx H where V is the volume worn away W is the normal load x is the sliding distance H is the hardness of the surface being worn away and k is a nondimensional wear coefficient dependent on the materials in contact and their exact degree of cleanliness. The term k is usually interpreted as the probability that a wear particle is formed at a given asperity encounter. Equation suggests that the probability of a wear-particle formation increases with an increase in the real area of contact Ar Ar W H for plastic contacts and the sliding distance. For elastic contacts occurring in materials with a low modulus of elasticity and a very low surface roughness Eq. can be rewritten for elastic contacts Bhushan s law of adhesive wear as Bhushan 1990 V k Wx Ec ap Rp 1 2 where k is a nondimensional wear coefficient. According to this equation elastic modulus and surface roughness govern the volume of wear. We note that in an elastic contact though the normal stresses remain compressive throughout the entire contact strong adhesion of some contacts can lead to generation of wear particles. Repeated elastic contacts can also fail by surface subsurface fatigue. In addition as the total number of contacts increases the probability of a few plastic contacts increases and the plastic contacts are specially detrimental from the wear standpoint. Based on studies by Rabinowicz 1980 typical values of wear coefficients for metal on metal and nonmetal on metal combinations that are unlubricated clean and in various lubricated conditions are presented in Table . Wear coefficients and coefficients of friction for selected material combinations are presented in Table Archard 1980 . Table Typical Values of Wear Coefficients for Metal on Metal and Nonmetal on Metal Combinations Metal on Metal Condition Like Unlike Nonmetal on Metal Clean unlubricated 1500 1 0 15 to 500 loi 6 10 - 6 Poorly lubricated 300 3 to 100 Average lubrication 30 to 10 .
