September AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | S E P T E M B E R 2 0 1 8 2 0 USING PHYSICAL VAPOR DEPOSITION TO OPTIMIZE SURFACE PROPERTIES Thin film cathodic arc PVD coatings offer an easy and economical way to optimize surface properties without changing bulk performance. David Bell and Viktor Khominich, Phygen Coatings Inc., Minneapolis Steve Midson, The Midson Group, Denver M any engineering components re- quire surface properties that are significantly different than bulk material properties. For example, supe- rior wear resistance might be required on the surface, while the bulk material might call for easy machinability. Often, both properties are not attainable in one material, necessitating a compromise of specifying a material with only moderate wear resistance and moderate machin- ability to make the component. Surface engineering technologies, however, al- low the properties and performance of a component’s surface to be modified and controlled independent of its bulk. There are a number of different surface engi- neering technologies available, includ- ing weld overlay, surface hardening, and hard coatings, but physical vapor depo- sition (PVD) offers a straightforward and inexpensive approach to optimize sur- face properties without changing bulk performance. PVD SPECIFICS Physical vapor deposition involves vaporizing atoms from a solid source and transporting and depositing them onto a substrate. PVD can produce dif- ferent categories of coatings includ- ing single elements such as titanium and diamond-like carbon (DLC), alloys (e.g., Al-Cr), and compounds (e.g., CrN and TiC). The most commonly used PVD coatings are metal nitrides such as CrN, TiAlN, and AlCrN, which are produced by admitting low-pressure nitrogen gas into the PVD chamber. This enables va- porized metallic atoms to react with the nitrogen gas during deposition on the substrate. Typically, PVD coating pro- cesses are performed at temperatures below 600 ° C, which enables depositing the coating onto a fully hardened steel without compromising the hardness of the steel substrate through over-tem- pering. PVD coatings are typically 1 to 6 µm thick. Cathodic arc evaporation (CAE), a commonly used PVD process, in- volves evaporation and ionization of at- oms from the target (source) material through the use of a high current-densi- ty arc. CAE produces high density coat- ings with extremely high adhesion and cohesion. However, one drawback of CAE is ejection of relatively large mac- roparticles (about 2 to 10 µm diame- ter) from the target material, which can become incorporated into the coating (Fig. 1a). Macroparticles form when un- wanted droplets of liquid metal splash from the arc source and land on the substrate during coating growth. As the size of these macroparticles is similar to the PVD coating thickness, and be- cause they are poorly adhered to the substrate, they negatively impact the coating’s integrity, performance, and overall life. This article describes a modi- fied cathodic arc process called arc plasma acceleration (APA), which en- ables production of high density, thin film coatings containing minimal macroparticles. (b) (a) Fig. 1 — (a) CrN coating produced using conventional cathodic arc evaporation (CAE) process contains detrimental metallic Cr macroparticle inclusions (white) and dark pores of various sizes; (b) CrN coating produced using the arc plasma acceleration process is essentially free of macroparticles.