Projects / Programmes source: ARIS

Self-lubricating and wear resistant PVD hard coatings based on (V,Cr,Al,Ti)N for hot-working processes

Research activity

Code Science Field Subfield
2.04.01  Engineering sciences and technologies  Materials science and technology  Inorganic nonmetallic materials 

Code Science Field
T155  Technological sciences  Coatings and surface treatment 

Code Science Field
2.05  Engineering and Technology  Materials engineering 
PVD coatings, hot forging, solid lubricant films, wear, friction
Evaluation (rules)
source: COBISS
Researchers (14)
no. Code Name and surname Research area Role Period No. of publicationsNo. of publications
1.  18271  PhD Miha Čekada  Materials science and technology  Researcher  2013 - 2016 
2.  18635  Tatjana Filipič    Technical associate  2013 - 2016 
3.  33428  PhD Peter Gselman  Biotechnology  Junior researcher  2013 - 2014 
4.  12616  PhD Darinka Kek Merl  Materials science and technology  Researcher  2013 - 2016 
5.  15703  PhD Janez Kovač  Electronic components and technologies  Researcher  2013 - 2016 
6.  23893  Marko Kuk  Manufacturing technologies and systems  Researcher  2013 - 2016 
7.  23883  MSc Vojko Leskovar  Mechanical design  Researcher  2013 - 2016 
8.  07642  PhD Vojteh Leskovšek  Materials science and technology  Researcher  2013 - 2016 
9.  15603  Andrej Mohar    Technical associate  2013 - 2016 
10.  26463  PhD Matjaž Panjan  Electronic components and technologies  Researcher  2013 - 2016 
11.  09090  PhD Peter Panjan  Materials science and technology  Head  2013 - 2016 
12.  15269  PhD Bojan Podgornik  Materials science and technology  Researcher  2013 - 2016 
13.  23882  Robert Ribič  Manufacturing technologies and systems  Researcher  2013 - 2016 
14.  15604  Tomaž Sirnik    Technical associate  2013 - 2016 
Organisations (3)
no. Code Research organisation City Registration number No. of publicationsNo. of publications
1.  0106  Jožef Stefan Institute  Ljubljana  5051606000  18 
2.  0206  Institute of Metals and Technology  Ljubljana  5051622000 
3.  1978  UNIOR D.D. Forging industry  Zreče  5042437 
In the last decades there has been an extreme progress in enhancement of cutting tool lifetime using hard PVD coatings. However, the problem of an effective protective coating for hot forming (hot forging, hot extrusion and die casting) remains unsolved. These processes are being used in several companies in Slovenia (Unior, Impol, Unitech, Rotomatika). Among these, hot forging has the longest tradition; it is used in the company Unior, Zreče (beneficiary of this project) for production of automotive components. Hot forged steel products have excellent mechanical properties, while the process disadvantage is low tool lifetime. Within this project we will be engaged in protection of tools for hot forging. The hot forging is distinguished by an extremely undesired combination of large mechanical, thermal and chemical loads, which cause abrasive and adhesive wear of the tool steel, as well as a crack network (due to thermal and mechanical fatigue). Using classical PVD coatings, the abrasive and adhesive wear can be reduced, but the onset and expansion of microcracks cannot be prevented. In the previous years an idea appeared to exchange classical lubricating liquids by self-lubricating hard coatings, deposited on the tool surface. Using such a coating, friction is reduced (which is one of the principle reasons for tool heating), abrasive wear resistance is enhanced, and tool surface temperature variations are reduced. The selection of coatings which fulfill these demands is not very large. Current research is primarily directed towards V-containing hard coatings. During hot forming, the tool surface heats up to a temperature high enough (around 600 °C), to initiate the diffusion of vanadium up to the coating surface. There the V2O5 forms which has excellent lubricating properties. After that phase the tool cools down to some extent, therefore the diffusion process stops. This mechanism is effective until all availableV has been consumed. In understanding the lubricating mechanisms, there are several unsolved problems: a) which coating is the most adapted for V doping; b) which is the optimum V concentration in order to ensure effective and long-lasting lubrication, while not degrading too much the hardness and other tribologically properties. In the scope of this project we will analyze sputter deposited multicomponent PVD coatings based on (Cr,Al,Ti,V)N, either in forf of a single-layer or a nanolayer. A sputtering apparatus with four magnetron sources will be used for deposition. In two sources segmental targets made of two triangular parts will be mounted. Each target segment will be made of a different material (V, Cr, Al). The coating composition will therefore have a gradient along the vacuum chamber axis. In this way, in one batch we will deposit a series of coatings with different compositions on a set of H11 tool steel samples. We will measure the influence of coating composition on structural and microstructural properties, microhardness, adhesion, oxidation resistance, friction coefficient and wear. Based on these data we will determine the optimal properties of the (Cr,Ti,Al,V)N coating. Tool steels H11 are hardened to around 48 HRC therefore they have not adequate loading support for the relatively thin hard and brittle PVD coatings. The loading support of tool steels can be substantially improved if they are plasma nitrided. An additional improvement is the compressive stress in the surface of the plasma treated tool steel, which prevents the emergence and propagation of microcracks. Therefore our test plates will be plasma nitrided prior to the PVD coating deposition. Before these steps, the nitriding process will have to be optimized, and the heat treatment of the steel too. This part of the project will be conducted at the Institute of metals and technologies. The duplex coating with the desirable composition will be tested for hot forging in real industrial production of the company Unior.
Significance for science
Today an effective wear protection of tools for hot forging is still a big challenge for technologists and scientists. During this manufacturing process the tools are submitted to very high thermal and mechanical cyclic stresses as well as to intensive friction and erosion. This finally leads to heavy damages of the tool surface due to wear, erosion, plastic deformation, thermal and mechanical fatigues. In order to extend the tool lifetime the tool surfaces are often surface treated. One of the most perspective ways for protection of forging dies is the duplex process, which combines plasma nitriding and PVD deposition processes. The nitrided layer improves the load-carrying capability of the steel substrates and also improves the thermal fatigue resistance. PVD coating can reduce the friction coefficient, increase the resistance to abrasive wear and improvements the thermal resistance. The use of conventional lubricants (graphite suspension in water) to reduce friction and wear is very important in hot forging process. The use of such lubricant causes the thermal fatigue cracking on die surface because it is subjected to thermal shocks. The use of lubricant means an additional cost to the manufacturing process and it is also environmentally unfriendly. These are all the reasons for replacement of conventional lubricating & cooling liquids by solid self-lubricating hard coatings, deposited on the tool surface. Beside good lubrication properties the potential coating for protection of hot forging tools must to retain an adequate mechanical integrity at high-T. Under hot forging conditions conventional hard coatings are not appropriate mostly due to the lack of good lubrication properties at high-T. Most promising coatings that fulfil these requirements are vanadium-based, self-lubricating, PVD hard coatings. Vanadium oxide is known to have good lubricating properties at high-T. The low-friction effect is attributed to liquid lubrication due to the formation of V2O5 with easily shear able planes (typical for Magneli phase oxides) and a low melting point (around 670 °C). The diffusion of vanadium to the top of the coating is a necessary condition, which provides continuous, self-lubricating effect during the operation of the coated tools. However, the out-diffusion of V should not be too fast, while a too intensive diffusion of V deteriorates the mechanical integrity of the coating. From this point of view, a simple VN hard coating is not appropriate because of rapid oxidation at elevated temperature. Different coating composition or architecture have been recommended in order to control the out-diffusion of V. One possibility is the incorporation of vanadium into conventional hard coatings (e.g. CrN, TiN, TiAlN). Another possibility is the deposition of different VN-based multilayer coatings (e.g. CrN/VN, TiAlN/VN). In both cases temperatures above 400–600 °C are needed to trigger the necessary out-diffusion of V and oxidation processes. In this project the application of the nl-structures composed from CrN, (Cr,V)N and VN layers was suggested. We found that at high-T the surface of such nl-coating is uniformly covered with a thin, chromium oxide layer, which acts as barrier for bulk diffusion of V. However we shown that the growth defects have an important role during the oxidation process. Such morphological irregularities provide fast tracks for the diffusion of V. Namely, the boundary region between the nodular defect and the coating matrix is very porous. We observed that in the early stage of oxidation the vanadium oxide is spread around the growth defects and later on it covers the whole surface. In this way we could retain sufficient mechanical integrity of the nl-coatings, while the diffusion of the V is slow enough to ensure the right amount of vanadium oxide to provide the low coefficient of friction. Such a combination of properties for hard coatings is demanded for the protection of hot forging tools.
Significance for the country
Unior company, the beneficiary of this project is ranked among the largest European forges. They exclusively supply the most demanding forged parts to top-quality manufacturers from the automotive industry. An improved product quality, higher productivity and reduced production cost are the basic guiding principles for further development of their forge programme. In order to achive this goals some improvements in all segemnts of production is necessary. Hence forging tools represents one of the most important segments of production of components especially for automotive industry. While the lifetime of tool plays a very important role in economy of hot forged parts, the prolongation of it is an important task of technologist in industrial production. Prolongation of tool life and prevention of its unexpected failure thus result in a lower cost per unit and a higher level of productivity. All processes of the forging die destruction (abrasive, adhesive and erosion wear, thermo-mechanical fatigue, plastic deformation) are located just in the surface layer of die material. Thus the effective way to improve the durability of forging tools is the modification of surface properties by e.g. plasma nitriding and by deposition of coatings having appropriate tribological properties (hardness, friction coefficient). Prolonged lifetime of the forging tool and reduced repair and maintenance means, a substantial impact on costs and productivity. In order to achieve higher competitive position on the market, the company Unior is looking for some new technology of improvement of hot forging tools. This project meets these challenges by development of new coatings that will be tailored for specific tribological properties to extended tool lifetime and reduce or even eliminate the use of conventional lubricating & cooling liquids. Thus we designed and developed of new type of hard self-lubricated nanolayer coatings based on vanadium. We expect that new surface engineering technology developed in the framework of this project will be exploitable commercially in the company Unior. Production volume within the forging industry is very high. Therefore we can expect that such innovative solutions which should increase tool lifetime and reduce tool maintenance costs will significantly increase the production process efficiency, product surface quality and thus have a considerable economic impact. Self-lubricant coatings developed in the framework of this project will have the following positive effects: • Extend the lifetime of forging tools • Reduced consumption of lubricating & cooling liquids • Reduce maintenance costs for tools • Improve the surface quality of the forging products. Thus the results of this project will enable the beneficiary to stay competitive in the next period. For successful use of coated forging tools in industrial production a complete understanding of the tribology mechanisms between the forging tool and the forming material is needed. Therefore we also performed in the framework of this project such kind of investigations on selected tools. Another aspect is also important. The PVD technologies are by any criteria ecologically acceptable. In comparison with conventional lubricating&cooling liquids the PVD self-lubricant hard coatings do not leave any hazardous waste, no wet chemicals, no aerosols or dust. The consumption of raw materials is minimal, and water is needed only for cooling and even this is minimal by using a closed loop. By scrapping the ecologically problematic chemical coating techniques and implementing PVD techniques instead the broader society benefits substantially. Last but not least results of this project and knowledge sharing widens the horizon of both partners: technologist from company Unior and scientist from the participating institute.
Most important scientific results Annual report 2013, 2014, 2015, final report
Most important socioeconomically and culturally relevant results Annual report 2013, 2014, 2015, final report
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