Most failures of power units occur due to the malfunction of regulators and distributors as well as plunger pairs of pumps and hydraulic motors. Unserviceable condition of plunger pair products of the same type is classified as a random event, i.e. a failure. In most cases, failures and destruction of plunger pair products occur due to wear and usually first appear on the surface of parts. All service properties of parts as well as their characteristics are closely interrelated with geometrical and physico-mechanical properties of surface layers. The work offers a probabilistic approach to solving the problem of assessing the quality of tribo-elements with wear-resistant coatings on the basis of diagnostic parameters of the acoustic testing method. It considers a methodology for determining an objectionable level as well as the risk value related to its choice in the process of testing tribo-elements with coatings. The work proposes different specific solutions to the problem of forecasting reliability of tribo-elements with wear-resistant coatings. In particular, it proposes mathematical models that provide the opportunity to estimate probability density for gradual failure of a tribo-element due to wear of the coating. The obtained analytic dependences make it possible to evaluate the probability of failure-free operation of tribo-elements, define the optimal value of coating thickness and forecast the life of a friction unit.