EFFICIENT PROCESSING OF HARD, BRITTLE MATERIALS

EFFICIENT PROCESSING OF HARD, BRITTLE MATERIALS Successfully competing in a global market requires a combination of having a range of unique advantages and ways of standing out from the crowd. Precision manufacturing offers this opportunity, but at the same time it poses challenges in terms of machinery, control and tooling. Six domains were identified in which a company can make the difference. SIX DOMAINS PROVIDING OPPORTUNITIES TO EXCEL Hard materials such as ceramics and cemented carbide are very useful in numerous components that are exposed to high mechanical, chemical and/or thermal stresses. However, these materials are not used as often as they could be as they are not only hard but also brittle. That means that they can break unpredictably. They are therefore slow and expensive to machine. But there are innovative solutions in the pipeline. Scientists are actively looking for ways to process brittle materials quickly and cost-effectively. Such as milling with very high precision, thanks to diamond-coated cutting equipment. Or by removing material with electrical discharges or vibrations. 1

MARKET NEED A higher wear resistance, a fatigue strength, a longer lifetime and a light weight are sought-after properties when examining hard materials, such as cemented carbide and ceramics, in product design. Although these materials would be ideally suited for this purpose, they are not used because of the difficulties in machining, which would create a high-cost, slow production process. To establish breakthrough cost-effective machining, technologies for both small and large series need to be introduced. 2

POTENTIAL & OPPORTUNITY The challenges when machining carbides and ceramics are, on the one hand, ensuring the required level of precision and, on the other, keeping down machining costs. When hard and brittle ceramic material gets chipped, this results in unpredictable dimensions, and a high level of tool wear and a low material removal rate add substantially to the price tag. However, the high levels of hardness and melting temperatures of hard and brittle materials make them ideal for a wide range of applications. In the automotive industry, high-quality technical ceramics are used so as to consistently comply with requirements that generally cannot be met by metal- or plastic-based materials. The l-sensor with doped ZrO2 as an electric conductor is a perfect example of this. Power stations use ceramic components in any machinery that is subject to high levels of mechanical, chemical and thermal stress. In the plastic industry too, components that face high levels of stress are made of high-quality technical ceramics. In other sectors as well, these materials can be found being used for various applications: mechatronics and semiconductors (bearings, precision components), the pump industry (valves, bearings, plungers), the food industry (valves, cutters), the chemicals industry (nozzles), the medical industry (hip balls, teeth, knives), aerospace (valves, sensors) and offshore navigation (guidance, bearings). Grinding is still the standard technology used for machining of hard and hardened materials. For ceramic components, an NNS will be produced in soft state (green part), followed by a sintering process which will lead to the shrinkage to an unpredictable extent of the dimensions of the relevant part, thus making a finishing grinding operation inevitable. For hard materials that are electrically conductive, EDM technology is also frequently used. However, nowadays new technologies are also available for machining free-form shapes 3

out of hard, brittle materials. The challenge is to incorporate these technologies into the production chain and achieving a cost-effective solution. RESEARCH RESULTS The paragraphs that follow will discuss various existing and promising prospective alternatives to grinding and EDM for machining cemented carbides and ceramics. High-precision milling High-precision milling allows machining of cemented carbide parts to take place without there being the drawbacks of both grinding and/or EDM. It thus improves the level of geometrical freedom (in the form of ensuring, within feasible limits, high surface quality and short lead times as a result of direct milling). To enable milling, a very hard diamond coating is required at the cutting edge (HV10 > 9,000 kg/mm 2 ), combined with new milling strategies. Other considerations that need to be taken into account when milling cemented carbide are a high level of machine stability; avoiding spindle expansion; minimising tool overhang; cooling due to compressed air; and high spindle speeds (30,000 rpm for a 2-mm diameter tool). However technological feasible it is though, the economics of milling cemented carbides mean that it not suited to all applications: tool cost (approx.. 250/tool diameter of 2 mm), dimensional limits (only small sizes) and a limited Material Removal Rate (MRR) of 2.2 mm 3 /min). High-precision (micro-)milling of ceramics is even more difficult. Experiments with a 1-mm end mill with different hard coatings and CBN showed that a nanograin diamond coating is most appropriate. With cutting parameters (ap = 4 µm, fz = 3 µm and vc = 120 m/min), a roughness of less than 60 nm and an MRR of 0. 912 mm³/min over a distance of 2 m are achieved. Delamination of the coating limits tool performance. The hard carbon coating in the tool breaks up above 4

Micrograin diamond (DM) Nanograin diamond (DN) Hard Carbon (HC) Coating material CVD diamond crystalline CVD diamond nanocrystalline Ta-C (>80% sp³) Coating technology CVD CVD PVD-Arc Coating morphology Crystals of 1-2 µm Crystals of 30-40 nm - Hardness (HV 0.05) 10.000 10.000 7.000 Max. service temp. ( C) 600 600 500 Coating thickness 6-10 µm 6-12 µm 0,3-1,2 µm Different coatings for milling ceramics 66 mm. The micrograin diamond coating has an intermediate performance, which can be explained by the larger size of the diamond crystals. With CBN tools and cutting parameters (ap = 10 µm, fz = 2-10 µm and vc = 60 m/min), a roughness of less than 150 nm and a maximum cumulative milling length of 341 mm was achieved. This process is mainly restricted in practice to finishing operations and micro applications. Electrical discharge machining (EDM) Electrical discharge machining is a machining technique in which material is removed by controlled electrical discharges (sparks) between an electrode (a wire or die in most cases) and a workpiece, both of which are submerged in a dielectric fluid. The EDM process imposes one big limitation on the workpiece material: it has to be sufficiently conductive. By adding a secondary conductive phase (e.g. WC, TiB2, NbC and TiN) to a non-conductive matrix (ZrO2, Al2O3, Si3N4), EDM can be enabled by creating a percolating conductive network inside the composite material. This network can form between 30 and 40 vol.% of conductive secondary phase and is dependent on the grain size, with smaller grains giving better results. As well as allowing EDM, the creation of a composite material has a positive effect on the mechanical properties, such as strength and fracture toughness. 5

Unlike metals, in which melting and evaporation are the main material removal mechanisms (MRMs), in ceramics and ceramic composites, other phenomena can play an important role in determining the material removal rate, electrode wear and surface quality. Grains can be dislodged during EDM due to differences in thermal expansion coefficients between the two phases, a process called thermal shock. This in general leads to a high level of surface roughness and a low level of tensile strength. Another similar MRM is spalling, in which a newly formed recast layer (a layer of molten material) breaks loose during the cooling process following a spark due to thermal stresses. In general this also leads to higher levels of surface roughness. A number of materials also undergo chemical reactions at the elevated temperatures occurring during EDM. Si3N4, a popular technical ceramic, does not melt but decomposes above 1,800 C into Si metal and N2 gas. The main MRM in WC is oxidation, leaving a nanometric WO3 layer behind which can be easily removed by warm water. In general these MRMs occur simultaneously, but most of the time one is dominant. Thermal conductivity is the main material property behind the material removal rate (MRR) when melting and evaporation are the dominant MRMs, due to the fact that a lower thermal conductivity ensures that the heat is concentrated at the surface during EDM. However, fracture toughness and grain boundary strength become important when thermal shock and spalling occur, and grain size appears to exert a strong influence on the oxidation behaviour. Therefore, when machining ceramics by means of EDM, knowledge of the interactions between material properties and machining behaviour is crucial when trying to achieve optimal MRRs and surface quality. Electro-chemical machining (ECM) ECM is similar to EDM, but there are a few major differences: no melting or evaporation takes place; no stress deformations are introduced, material removal occurs by means of dissolution of the anode 6

(workpiece) (MRR = 1.5 cm³/min), an electrolyte is used to transport ions between the cathode and the anode and often the electrode vibrates to improve evacuation of the electrolyte. An electrically conductive part can be machined regardless of its hardness down to a very low surface roughness. Due to the very narrow gap between the cathode and the anode, parts can be machined with very high precision and very low Ra values (approx. 20 nm). Vibration-assisted machining In vibration-assisted machining, a vibration is added to the cutting tool. This can be a turning tool, a drilling tool or a grinding/milling tool. Typically, the amplitude ranges between 1 and 40 µm, and the frequency can be anything up to 80 khz. At frequencies between 18 and 25 khz, the process is known as ultra-assisted machining. The vibration significantly increases the ability to machine hard and brittle materials (SiC, Al2O3, ZrO2, etc.). The vibrating movement has the advantages of lower cutting forces which allow higher material removal rates, and the machining of small details. The vibration also results in better cooling conditions, and the different wear mechanisms have a tool sharpening/dressing effect. Electrolytic in-process dressing (ELID) grinding ELID grinding enables the use of metal-bonded grinding wheels which are very durable and difficult to deal with in comparison with resinous and vitrified wheels. Furthermore, with ELID grinding, it is possible to obtain both good geometrical accuracy and a very smooth surface on a workpiece (because of very small abrasives of 1 µm). Several types of cemented carbide (WC-Co) and ceramics (ZrO2, Si3N4, etc.) have been ground with ELID to a surface roughness Ra of a few nanometres. This results in mirror-like surfaces of very polished quality. You will find below two images of ground workpieces: 7

ELID ground WC-Co ELID ground ZrO2 the WC-Co piece on the left measures 40 x 50 mm, has a surface roughness Ra of 0.007 µm (Rz of 0.063 µm) and a straightness error of 1 µm over both its width and its length. On the right-hand side is a piece of ZrO2 of 50 mm in length and 20 mm in width which has a roughness Ra of 0.006 µm (Rz of 0.050 µm) and a straightness error of 0.6 µm over its length. Laser-assisted turning In laser-assisted turning, a laser beam locally heats up the workpiece material just before cutting. Locally heating this material improves the machinability of high-strength materials like ceramics. Besides better machinability, laser-assisted machining holds out several benefits: higher cutting volumes and longer tool life (MRR of up to 10 mm 3 /s), a shorter manufacturing time and lower costs, the elimination of cooling lubricants (dry machining), geometrical flexibility, affordable manufacturing of complex components made from technical ceramics, and a highly reproducible manufacturing quality due to a very good level of control of the laser source. Laser-assisted turning on Monforts RNC LaserTurn 8

Turning Laser assisted turning Parameters vc [m/min] 30 60 F [mm] 0.01 0.01 Ap [mm] 0.17 0.42 Laser power [W] 0 2000 Material removal rate [mm 3 /s] 0.86 4.31 Results Ra 0.08 0.10 Rz 0.60 0.67 Tool wear VBmax [µm] 40 46 A comparison between turning and laser assisted turning of ZrO2 Material E [GPa] Hv [kg/mm³] KIC 10kg [MPam0.5] K [W/m K] ρ [10-5Ωm] ZrO2 - TiN 280 1350 9.7 6.41 2.94 DIe sinking EDM Milling ELID-Grinding UAG Micro-EDM Material removal rate [mm 3 /s] Surface roughness (Ra) [µm] Shape flexibility Turning Laser assisted turning UAG Technology ELID-grinding Diesinking EDM Micro EDM Micro milling 1.18 0.86 4.31 1.66 0.15 0.0002 0.0152 2.2 0.020 0.08 0.10 0.2 0.65 0.5 0.03 0,06 Milling Low Medium Medium High High Medium High High An example providing an overview of the techniques above 9

Laser ablation Laser ablation is a mass removal technique involving coupling laser energy to a target material. It is already widely applied in metal-cutting processes. However, for ceramics, due to their extremely high melting and boiling points and high thermal conductivities, the efficiency of conventional lasers is limited. The development of femtosecond lasers holds out new opportunities in terms of the laser machining of ceramics with an extreme high melting point. Femtosecond lasers (10-15 s) allow for a non-thermal ablation regime (τ pulse < T thermal ) yielding a reduced dependency on the thermal properties of the target material. Instead of melting/evaporation, the material removal mechanism is adsorption/excitation of the target material at a rate faster than heat is conducted inside the material. In addition to enabling ceramics to be machined, the non-thermal machining mechanism of femtosecond lasers considerably increases the surface quality and precision of laser-machined parts. 10

INDUSTRIAL EXAMPLE To demonstrate the milling of carbides, a hexalobular-shaped punch die has been produced (see the image below). The pocket, which has a diameter of 9 mm and a depth of 6 mm, is machined in cemented carbide WC/Co 90/10 to a precision level of 0.02 mm. Two diamond-coated micro ball-end mills were used, one for roughing and one for finishing, both of them 1 mm in diameter. The roughing operation took about 156 minutes, and the finishing operation 44 minutes at 20,000 rpm and a feed rate of 200 mm/min. The cutting parameters can be set even higher, since while there was tool wear, this was at a rather low level, but in this case the limits of the machinery were reached. The table above shows that it is possible to machine the punch die but the economics of the process need to be looked at. Tool costs, at about 250/tool, are rather high. Alternative technologies, like wire EDM, can combine multiple parts in one operation, if the shape of the product allows this. However, as the number of applications increases, surface tool costs will inevitably go down and the milling of carbides will become competitive for a wide range of such applications. Carbide hexalobular-shaped punch 11

SEIZING THE OPPORTUNITY Machining hard and brittle materials is no longer the exclusive preserve of grinding. When looking to increase cost-effectiveness and geometrical flexibility, a variety of novel technologies are available. However, it is important to set up the correct combination of technologies to achieve a cost-effective production chain. The key here is to remove as much material as possible as fast as possible in the roughing stage and so reach an optimal starting point for the more expensive finishing operation. For each product the make-up of this production chain has to be examined. 12

EXPERTISE AND FACILITIES AT YOUR DISPOSAL The Precision Machining Lab at Sirris: the Fehlmann Versa 825 five-axis high-precision milling centre; the high-precision Erowa clamping system; the Mitutoyo Apex-S 3D coordinate measuring machine; a laser texturing machine for surface functionalization an acclimatised chamber. Various specifications: milling of precision components to an accuracy of 3 μm; machine travel range: X: 820 mm; Y: 700 mm; Z: 450 mm; spindle: 20,000 rpm, 24 kw and 120 Nm at 50-1,920 rpm; clamping with micrometric repeatability; CNC-controlled (scanning) measurements from CAD; measurement accuracy of 1.7 μm + 0.3 L/100 μm (L in mm). The precision machining lab, its infrastructure and engineers, are at your service to: realise your prototype precision components for new applications; become conversant with precision machining before investing yourself; provide you with support with regard to the machinability and cost-effective manufacturing of precision components. 13

THE AUTHORS Sirris is the collective centre for the Belgian technology industry. The Advanced Manufacturing Department boasts more than 60 years of experience in the field of machining technology. Sirris was the first organisation in Belgium to introduce NC programming, damped-boring bars, tool management, high-speed milling, five-axis simultaneous milling, hard turning and laser ablation. Over the last four years the focus has been on achieving micrometric precision levels on five-axis milling machines that, while high-end, is within the reach of SMEs. Working with industry, our applied research has led to game-changing results. Peter ten Haaf Program Manager - Precision Manufacturing As responsible for the Precision Manufacturing department Peter defines the research strategy and supports industry in detecting their own opportunities. Olivier Malek Expert Machining Advanced Materials and Surface Functionality Olivier is responsible for research and industrial projects on high precision machining. His interests lay in non-traditional machining technologies and advanced materials in particular. Krist Mielnik Expert High-precision Milling Krist focuses on the finishing process optimisation of the gear prototype, realignment problems and precision finishing of additive manufactured parts and methods to evaluate and improve machine precision. Tom Jacobs Expert Machining Advanced Materials and Monitoring Solutions As a senior engineer, Tom is helping companies with research on methods to control precision during production with the help of sensors and real-time data. 14

PARTNERS The research descripted within this publication was a collaboration between This publication has been made within the framework of VIS and supported by Agentschap voor Innovatie door Wetenschap en Technologie (IWT). DIAMANT BUILDING Boulevard A. Reyerslaan 80 B 1030 Brussel +32 2 706 79 44 www.sirris.be info@sirris.be blog.sirris.be 15