Corrosion Behavior and Surface Properties of PVD Coatings for Mold Technology Applications

Abstract

Chrome plating is still one of the best solutions to coat martensitic steel used in the molding of plastics and rubbers. However, current stringent regulations on environmental impact call for more sustainable processes. In the present work, some physical vapor deposition (PVD) nitride coatings were produced on X155CrMoV12 steel and characterized in terms of both corrosion behavior and surface properties. Results indicated that titanium-based PVD coatings could be a valuable alternative to chromium-based coatings as they exhibited a good compromise between corrosion and surface properties. AlTiN and TiN PVD coatings exhibited adequate hardness for plastic mold applications, with AlTiN reaching hardness as high as 2000 HV. Moreover, the critical loads and adhesion properties were found to be definitely better than those of chromium-based coatings. From a corrosion point of view, the presence of multilayers in AlTiN did not seem to be beneficial as the breakdown potential for TiN (single layer) was ca. 1.1 V vs. saturated calomel electrode (SCE) compared to 0.85 V vs. SCE for AlTiN in aggressive media (NaCl).

Insight into the Release Agents/PVD Coatings Interaction for Plastic Mold Technology

“…It is visible that NbN and, even more, AlTiN are multilayers coatings, whereas ZrN and TiN display a dense structure without clear imperfections. Furthermore, no configuration shows any cracks or delaminations at the interface, thus appearing adherent, compact and uniformly distributed along the surface profile [14]. All the coatings possess a micrometric thickness, notably in the range between 1.4 µm for TiN and 3.7 µm of ZrN.…”

“…Nowadays, among several mold materials, nitride-coated (e.g. TiN, AlTiN and CrN [5,14,15]) metal substrates, obtained by Physical Vapor Deposition (PVD) method, show improved mechanical properties and good wear/corrosion resistance [14,[16][17][18].…”

“…On the contrary, as concern the second modus operandi, the release agents allow the reduction of mold fouling; they are deposited as a very thin layer to the mold surfaces, covering them uniformly and acting as a barrier that prevents the rubber sticking.Nowadays, among several mold materials, nitride-coated (e.g. TiN, AlTiN and CrN [5,14,15]) metal substrates, obtained by Physical Vapor Deposition (PVD) method, show improved mechanical properties and good wear/corrosion resistance [14,[16][17][18].The surface wettability by a liquid is defined in terms of both spreading coefficient (S) and work of adhesion (W a ), by evaluating both the surface free energy of substrate and the surface tension (γ) of the liquid. Thus, if these two proprieties are similar, the mold release spontaneously wets the surface.…”

“…The samples used in this research study were made by Vacuum Cathodic Arc [17]: this technique is one of the several methods to obtain nitride/carbide-based PVD coatings of different metals (Cr, Zr, Ti, Mo). A martensitic stainless steel (X155CrMoV12, K110 Bohler stee, DIN 1.2379, AISI D2) was used as the coatings substrate, with a nominal chemical composition of C 1.55%; Si 0.30%; Mn 0.30%; Cr 11.5%; Mo 0.70%; V 1.00% [14] (see Figure S1). All the PVD coatings (AlTiN, NbN, ZrN and TiN) were produced by CRT company (Nerviano, Milan, Italy) and prepared by means of cathodic arc deposition technique under controlled temperature (350 • C for NbN; 450 • C for AlTiN, ZrN and TiN) and pressure (1.5 Pa for NbN; 2.5 Pa for AlTiN, ZrN and TiN).…”

“…In the case of NbN and AlTiN, the final coatings were composed by multilayers, 4 (Cr, CrN, CrN+NbN and NbN) and 8 (TiN, AlTiN, 5 subsequent layers of AlTiN+TiN and lastly AlTiN one), respectively; TiN and ZrN, instead, were deposited as a single layer. In a previous work [14], similar PVD coatings have been characterized and studied concerning either the mechanical features (such as the hardness and the critical load) or the corrosion resistance in different environments.…”