Metal forming members such as dies and the like are exposed to high cyclical pressure and high friction conditions. Recently the industry has turned to the use of advanced high strength steel alloys (AHSS) which exhibit tensile strengths far greater than those shown by previously employed steel alloys. Typical AHSS alloys manifest tensile strengths which exceed 700 MPa. In particular instances, alloys having tensile strengths in the range of 900-1200 MPa are employed in the fabrication of structural components for automobiles and the like. Owing to their high strength, AHSS steels are very difficult to form and dies used for their forming are exposed to high impact and high pressure conditions. In addition to the high impact and high pressure conditions, the surfaces of the dies experience very high frictional forces in use. These high pressure, high friction conditions can cause extreme wear to forming dies and the like, thereby greatly compromising their service life.
In some instances, the prior art has sought to harden the surfaces of the dies by various treatment processes; however, these results have produced only limited success. In other approaches, dies have been coated with various high hardness, wear resistant coatings including titanium nitride, titanium carbonitride, chromium nitride, titanium aluminum nitride, and the like. In other approaches, thermal diffusion coatings of materials such as vanadium carbide and chemical vapor deposition (CVD) coatings such as titanium carbide have been utilized. While each of these surface enhancement techniques have demonstrated success in extending the service life of dies used to form conventional alloys having strength levels below 400 MPa, they all manifest poor performance in forming operations involving AHSS alloys.
In some instances, thermal diffusion and CVD coatings have been found to extend the service life of dies used for forming AHSS alloys. However, both of these processes tend to produce changes in the dimensions of the dies resultant from changes in crystal structure during the deposition and heat treatment of commonly used tool steel die materials. The magnitude of these dimensional changes is very often outside the dimensional specifications defined by the users of the forming dies; hence, such processes are generally not acceptable. Further, thermal diffusion coatings have relatively high coefficients of friction and are incompatible with the high levels of frictional forces generated in the forming of high tensile strength AHSS materials.
While plasma vapor deposition (PVD) coatings, unlike CVD coatings and thermal diffusion coatings, do not cause unacceptable dimensional changes to the forming dies, many of such coatings have other limitations which restrict their use in high tensile strength material forming applications. Because of their columnar structure, PVD coatings such as chromium nitride, titanium aluminum nitride, and the like are susceptible to cracking when subjected to high pressure/impact conditions such as those found in the forming of materials having tensile strengths greater than 400 MPa. Some increase in die performance has been achieved by case hardening the surfaces of the dies prior to coating with conventional PVD coatings since the increased surface hardness of the die prevents deformation and cracking of the coating. But, this approach produces only limited success and is useful only in those instances where the tensile strengths of the steel alloys being formed are below 800 MPa.
As a consequence, there is a need for coatings and methods for improving the service life of metal forming members such as dies, used in connection with the shaping of AHSS alloy materials. Such coatings should be durable under very high pressure and high friction conditions encountered in the forming process and should not adversely affect the dimensional parameters of the die. In addition, processes for applying the coatings should be simple, economical, easy to implement, and recoatable. As will be explained in detail hereinbelow, the present disclosure provides a composite coating for metal forming members which achieves the foregoing goals.