Domestic research projects


Research projects (co)funded by the Slovenian Research Agency.


  • Member of University of Ljubljana: UL Faculty of Mechanical Engineering
  • Project code: L2-1836
  • Project title: Development and implementation of innovative machining technology for machining ZnO based ceramics with defined cutting geometry, in serial production, to increase the quality of varistors as final products
  • Period: 01.07.2019 - 30.06.2022
  • Range on year: 0,65 FTE
  • Head: izr. prof. dr. Franci Pušavec
  • Research activity: Engineering sciences and technologies 
  • Research Organisation: Link
  • Researchers: Link
  • Citations for bibliographic records: Link

A little less than 90% of new energy produced in the EU is the energy from renewable sources. From year to year, European countries invest record inputs for new investments (in 2017 alone, they amounted for 51.2 billion euros). Harnessing renewable energy sources is one of the largest investment opportunities in the energy sector. The field of wind energy exploitation accounted for half of all investments in 2017. Today, wind power plants provide 11.4% of total demand for electricity in the EU. Among all European countries, Germany covers the largest share and at the same time predicts the closure of all nuclear power plants by 2022. Other EU members also follow this trend which means that renewable sources and their production (wind power plants, etc.) are strongly on the rise both in the terms of investments and numbers of new workplaces. 

Department for Management of Production Technologies Laboratory for Machining, cooperates with the company VARSI, which with its innovative and unique approach penetrates into a brand new niche market, i. e. ZnO varistor energy absorbers (VEA-Varistor Energy Absorbers). These are specially designed varistors for the absorption of large energies to protect electronic and electrical systems in renewable energy sources systems. They are primarily used to protect windings of generators and inverters in wind power plants with power ratings from 3 MW up to 8 MW and more recently to protect devices in photo voltaic systems. The varistor is a voltage-dependent resistor characterized by a nonlinear voltage-current dependence. When functionally integrated it is an insulator to a certain voltage but when this voltage limit is exceeded, it becomes highly conductive. When the varistor is exposed to a sudden voltage surge (at a given current density) the resistance changes in a very short time (a few nano seconds) from very high (1012 Ω) to a very low value (1-10 Ω). Due to this property the varistors are used to protect circuits from overvoltage and current shocks. Due to the non-linear voltage-current (U/I) characteristics and the ability to absorb energy the varistors are used to stabilize the voltage and to suppress overvoltage gaps in power grids and in electronics. The automotive industry and telecommunications services (GSM devices) are also large users of varistors. The energy efficiency of these absorbers in the absolute sense is in the range from a few 10 kJ up to 200 kJ, depending on their size and number, but even more important is the value of the specific energy absorption per volume unit that they managed to improve from 0.4 kJ/cm3 to 0.6 kJ/cm3, that is, for a whole 50% and recently this value has reached 0.7 kJ/cm3 and more. Innovative prototype solutions of the company VARSI with the support of the development centre RC eNeM (development of new materials) place such varistors in the competition against the world's leading manufacturers. 

Industrial ceramics is a material with high hardness, porosity and fragility. The usability and the interest in the applicability to the engineering fields (automotive, aviation, electrical energy, etc.) is increasing every year. In general, for the production of ceramics the sintering process is used for both oxide and non-oxide ceramic mixtures. For the ultimate functionality of the products, structure and both micro and macro geometry of contact surfaces are vital. This requires subsequent finishing. Due to the high hardness of these ceramic materials even at high temperatures, finishing machining technologies with undefined tool or cutting edge geometry are used. A typical example of such conventional treatment is the use of grinding or lapping with abrasive particles. Due to the need for high material removal rate and the requirements involving complex surface geometries of the product, the grinding technology represents limited productivity, inability to produce the required geometries and frequent uncontrolled chipping or sharpening of the finished products' edges. As an alternative, the research work will be focused on the implementation of the machining processes with defined cutting tool geometry (milling and turning) into the machining of ZnO ceramics. The preliminary experiments showed great potential of such technologies in improving the quality of the products. The problems that arise are connected with the chip formation mechanisms due to the material properties of ceramics (strenght and hardness). Therefore, the goal of this research work will be oriented in the analysis of the mechanisms of cutting and chip formation. As a research contribution, it is expected to link the microstructure of the workpiece, the geometry (radius) of the tool cutting edges and the change in the geometry of the tool as a result of tool wear. The new technology has been pilot tested and machined prototypes verified where the results confirm the achievement of the required geometric properties and the robustness of the production technology. The excellence of the prototypes is also reflected in the real electrical tests. Thus, the main goal and purpose of the project is the wider analysis of new production technologies of ceramic varistors based on milling and turning.  Thus, the goal is to implement the developed production technology into an industrial application and verify it on a specific product. In addition, the goal is to develop and raise the technology to the large-scale production of new high-quality varistors.

The phases of the project and their realization:
  • WP1: Preparation of specifications for equipping and upgrading machines and preparation of experimental system and site:

On the basis of acquired knowledge and process parameters in the WP1 we will be able to begin with the development of concept and later prototype which will be upgraded and developed, produced, tested and compared with products, manufactured by other production technologies (current technology – lapping). In addition to suitably selected CNC machines it will be also necessary to provide a suitable clamping system for clamping ceramic discs, suitable tools and tool holders, suitable laser assisted machining system (laser source, beam guide, suitable collimator) and finally the robotic arm for automatic manipulation with workpieces and finished products.

  • WP2: General research and analysis of the influence of varistor ceramics cutting process on the varistors edge radius, parallelism of surfaces, surface roughness and tool life:
    • varistor ceramics edge radius
    • quality of machined surface and
    • tool wear and tool life


  • WP3: Research of the laser assisted ceramics cutting process
    • Quality of the machined surface analysis and possible degradation of processed surfaces/products and
    • the application of a thermal model in order to predict the behaviour and set the quality guidelines
    • production, testing and optimization of the system and technology