A wide range of unique engineering structural and performance properties inherent in metallic composites characterizes wear- and erosion-resistant high-temperature coatings made by thermal spraying methods. This allows their use both in manufacturing processes to enhance the wear strength of products, which have to operate under the cyclic loading, high contact pressures, corrosion and high temperatures and in product renewal.

Thermal coatings contribute to the qualitative improvement of the technical level of production and product restoration using the ceramic composite materials. However, the possibility to have a significantly increased product performance, reduce their factory labour hours and materials/output ratio in manufacturing and restoration is largely dependent on the degree of the surface layer quality of products at their finishing stage, which is usually provided by different kinds of machining.

When machining the plasma-sprayed thermal coatings, a removing process of the cut-off layer material is determined by its distinctive features such as a layered structure, high internal stresses, low ductility material, high tendency to the surface layer strengthening and rehardening, porosity, high abrasive properties, etc. When coatings are machined these coating properties result in specific characteristics of chip formation and conditions for formation of the billet surface layer.

The chip formation of plasma-sprayed coatings was studied at micro-velocities using an experimental tool-setting microscope-based setup, created in BMSTU. The setup allowed simultaneous recording both the individual stages (phases) of the chip formation process and the operating force factors.

It is found that formation of individual chip elements comes with the multiple micro-cracks that cause chipping-off the small particles of material. The emerging main crack in the cut-off layer of material leads to separation of the largest chip element. Then all the stages of chip element formation are cycled. Fluctuations of the cutting force components completely repeat all the stages of local destruction and formation of individual chip elements. Studies have shown that with increased thickness of the cut-off layer the main crack develops below the cut-off line thus significantly affecting the quality of the machined surface: emerging cracks, cavities, chips, and other defects that significantly reduce the product performance.

In machining the plasma-sprayed coatings, their high tendency to strengthening and rehardening because of the cutting action has a great impact on the surface quality. This is evident as a loss of the elastic equilibrium state stability and as a destruction of the work-piece surface layer (chipping, peeling, flaking). With increasing curvature of the machined surface (e.g., decreasing radius of cylindrical billet) the surface layer is increasingly prone to destruction.

Specific problems are cutting fluids used in grinding the plasma-sprayed coatings. Machining in this case comes with saturated liquid vapours formed in the surface layer of a billet under high pressure. With a pressure drop on the tool-side in cutting there is such an intensity of vaporization that micro- and macro-fractures of the machined work-piece surface can be initiated.