Benefits of post-coating treatment: Turning Performance

Coatings for metalcutting tools have made tremendous advances since their introduction more than 40 years ago. As coatings have improved, our understanding of their behavior in a variety of cutting operations has increased, and this has led to further developments to enhance performance.

A technology that has made big strides in the past few years is post-coating treatment. As thicker coatings were developed and Al₂O₃coatings came into the industry, these treatments began as efforts to smooth the coating surface of CVD coatings. People observed that a smooth coating surface enhanced tool life by reducing both friction and the tendency of workpiece material to stick to the surface, which causes built-up edge. This could be called the first generation of post-coating treatment.

The surface-smoothing process is a light grit-blasting operation that removes the roughness caused by uneven coating-grain growth. Only a tiny amount of coating is removed. As these blasting processes have become more controlled, it has become possible to accurately remove a very thin layer. An example is removal of the “flash” TiN coating layer on the rake surface of an indexable insert. The advantage of the process is that it provides a smooth top surface, leaving a gold TiN layer on the flank for easy wear detection. This process could be called the second generation of post-coating treatment.

Wear on a coated insert after interrupted cutting of 54 parts (top), and wear on the same type of coated insert, but with a post-coating treatment, after interrupted cutting of 80 parts. Image courtesy of Walter USA.

Sometimes during coating, particularly during CVD processes, the coating layers can experience tensile stress. This state is caused by differences in the thermal expansion coefficients of carbide substrates and hard coating layers. A tensile-stress state can cause flaws in the coating to grow into cracks. It also tends to cause those cracks to further expand until they result in failure of the cutting edge. Most often, these cracks are generated from the stresses of the cutting operation, especially interrupted cuts.

Because coatings are ceramic materials, they are inherently weak at resisting tensile stresses but incredibly strong at resisting compressive stresses. Cracks and other flaws are unable to grow in a compressive-stress state. As second-generation treatments have been improved and controlled, it has became apparent that blasting has had the further beneficial effect of reducing the tensile stresses in CVD coatings.