ICMCTF2014 Session G5: Coatings, Pre-treatment, Post Treatment and Duplex Technology

During production processes hot forging tools are subject to complex load regimes (spectra), which are additionally acting localized depending on the geometry of the die. The resulting extensive wear is caused by simultaneously acting high mechanical, tribological, chemical and thermal cyclic loads.

The rapid failure of the form giving tool components is limiting the cost effectiveness of the processes. Hence there is a big interest in reducing the occurring wear.

Recent research results suggest the promising use of locally applied duplex processes consisting of plasma nitriding and plasma deposition techniques.

There were found process-related and geometrically depended wear mechanisms acting in selected zones of the dies. This can be the loss of the strength of the base material caused by annealing processes, abrasive and adhesive wear caused by the material flux and cracking in the surface near zone caused by mechanical and thermal cyclic loads. In order to increase the operating life of the forging tools there is a demand to adapt duplex treatments.

The nitriding as state-of-the-art treatment of forging tools shows an additional demand for further optimization. With respect to the appropriate load situation there are existing different possibilities in applying localized treatments by covering the tools mechanically. Thus, it is possible to obtain different intensities of the nitriding considering a two-step treatment. As a result, intense thermally loaded zones can be stabilized with intense nitriding while ductile material properties are maintaining crack sensitive zones.

The combination with newly developed vanadium doped chromium nitride or boron containing titanium nitride coating systems enables the reduction of abrasive and adhesive wear components. Both are thermally stable and reveal high hardness combined with friction reduction properties.

The presented duplex treatments are improving the properties of the surface and surface near zones of forging tools. Thus, this technology allows an economic and sustainable manufacturing process.

New data about the fracture toughness (K_C) of cobalt boride coatings (CoB and Co₂B) were estimated from depth-sensing Vickers microindentation technique. The formation of the CoB and Co₂B coatings on the surface of the CoCrMo alloy was performed by the powder-pack boriding process at temperatures of 1223-1273 K with different exposure times for each temperature. The mechanical characterization of the boride coatings was divided in two procedures: first, Vickers indentations were conducted at constant distances from the surface using loads ranging from 15 to 450 mN. For all sets of experimental conditions, five acceptable indentations were carried out for each applied load to establish the behavior of the hardness (H) as a function of the diagonal length (d) and to verify the presence of the indentation size effect (ISE) in the boride coatings. Based on these results, the apparent or real hardness (H_o) of the CoB and Co₂B coatings was determined using the deformation band (DB) model. Second, the crack lengths developed on the corners of the indentations marks (with applied loads above 250 mN) were measured on both coatings using a scanning electron microscope (SEM).

Considering the indentation results in the set of experimental conditions of the boriding process, the K_C of the cobalt boride coatings were estimated using a universal crack equation applicable independently of the cracking mode. The results indicated that the CoB and Co₂B coatings exhibited two types of cracking modes (intermediate and radial-median, respectively), in which the fracture toughness of the Co₂B coating was ten-fold greater than the CoB coating.

Large area surface modification and silicon dioxide deposition process parameters are explored for a new multi-function plasma source. The Dual Magnetron Plasma Source has demonstrated the ability to pre- treat several common polymer based substrates prior to vacuum deposition and deposit SiO2 films from HMDSO with high rates . The pre- treatment process results are reported and the process variables for the deposition process are statistically analyzed to determine the critical interactions.

The growing competitive pressure in the industry makes the reduction of process costs increasingly important. One possibility to decrease the costs is to increase the life time of the tools by applying of wear resistant and friction reduced PVD coatings. Especially in the field of hot forming, tools are exposed to high mechanical and thermal loads. The special requirements of these tools can be met by multilayer systems such as Ti/TiAlN or Cr/CrAlN, which provide properties such as a high hardness, sufficient heat resistance, and smooth surfaces. However, varying hardnesses and different thermoelastic properties between the multilayer systems and the substrate can lead to the delamination and spallation of the layers. Using an upstream plasma nitriding process, it is possible to produce a hardness gradient between the basis material and the coatings, thus ensuring a sufficient supporting effect for the coating. In this work, the nitriding parameters were successively varied and the influence of the parameters on the material performance in the nitriding zone was examined. Afterwards, the adhesion of the applied PVD layers was investigated in dependency on the nitriding parameters. The PVD layers were deposited on tool steel substrates AISI H11 (1.2343) by means of magnetron sputtering. The mechanical and tribological properties as well as the adhesion of these coating systems were studied using a scratch tester, a nanoindenter, and a ball-on-disc test.

Nitriding is the process of surface nitrogen saturation in metallic materials, which is performed to improve the fatigue, corrosion and/or tribological properties. The nitriding on the surface of ferrous alloys results in the formation of a compound layer of γ´ and ε types nitrides or a mixture of γ´ and ε with a nitrogen diffusion zone beneath the nitride layer. The nitride (compound) layer can be beneficial for the resistance against wear and corrosion. The diffusion zone brings about a strong increase of the fatigue resistance and also increases the wear resistance. The broad range of properties of the nitride layer required by different applications needs the control of the nitriding process. In order to get an optimal result all the process variables have to be under control.

The powder-pack nitriding is a process analogous to pack carburizing. It employs certain nitrogen-bearing organic compounds as a source of nitrogen, in which the compounds used in the process form reaction products that are relatively stable at temperatures up to 843 K.

In this study, new data about the growth kinetics of (γ´ and ε) layers developed during the powder-pack nitriding process on the surface of ARMCO pure iron were estimated. The powder-pack nitriding of the pure iron was performed according to the Pulnieren^© (H.E.F. Durferrit) method by using a “Pulnier”-powder and an activator, and was conducted at 773 – 848 K for different exposure times (2 – 12 h) for each temperature. In addition, according to the set of experimental variables of the nitriding process, three different “Pulnier” powder - activator mixtures (0.20, 0.25 and 0.30) were used to evaluate the activation level during the growth of the nitride layers.

The residual stresses before and after the deposition process of PVD coatings have a strong influence on the coating adhesion and lifespan of machining and forming tools. Therefore, the understanding and control of the system’s residual stresses will lead to a better performance of the coated parts. Moreover, investigations have been conducted in the field of stress analysis of PVD coatings, yet they were not specifically focused on the interdependency of residual stress states in the coating and the substrate.

In this investigation, three different metallographically prepared substrate pre-treatments were used. SiC grinding, Diamond grinding, and SiC grinding and plasmanitriding preparations were selected, due to the substantial difference in their final residual stress state. Additionally, Ti/TiAlN multilayer coatings and a reference TiAlN monolayer were deposited on each pre-treated substrate.

Their initial and final residual stress states were measured by X-ray diffraction. In addition to the residual stress analyses, tribo-mechanical tests were conducted in order to correlate the results with the identified residual stress states. In order to complement the tribo-mechanical measurements, nano-indentation, ball-on-disc, as well as scratch tests were performed.

PVD and PACVD coatings offer surface engineering solutions in many applications, like in the tool, tribological and decorative fields. The combination of the PVD/PACVD coating with other surface treatments can improve the performance of the coated component. An overview of examples for different applications will be given.

A plasma nitriding pre-treatment of forming tools (duplex treatments) will improve the load bearing capacity for the PVD coating. Edge honing of cutting tools allows thicker PVD coatings and can clearly improve the lifetime of the cutting tool.

A polishing intermediate treatment has been found to be advantageous to improve corrosion resistance. The effect of the treatment on the corrosion resistance of Cr/CrN coated steel will be shown.

Polishing post-treatments are a well-known finishing for arc evaporated coated cutting and forming tools to reduce the roughness, improve friction and chip removal. Examples of polishing of sputter/PACVD coated components to reduce running-in effects will be presented as well.