Free Sintering or Hot Pressing? A Decision Support

Transcription

1 Free Sintering or Hot Pressing? A Decision Support Christian H. Kühl Diamond Tool Consulting Neuhofer Straße 13b, Henstedt-Ulzburg christian.h.kuehl@gmx.de Abstract Free Sintering is, in classic powder metallurgy, a proven and common technology, hot pressing, the most common sintering process in the diamond tool industry, plays a minor role. In the following it will be shown what the reasons are and what needs to be done to give free sintering the importance it deserves. Price and competition pressure are forcing the tool manufacturers to ongoing cost savings. Free sintering is an effective possibility to lower the production cost significantly. This article deals with the physical differences of both processes such as the three dimensional shrinkage or the impact of different process parameters on the properties of a bond. It shows advantages and disadvantages of both processes regarding to capacity and flexibility and the cost saving potential of free sintering. 1 Introduction Free sintering or pressureless sintering are currently technical expressions which are used in the diamond tool industry to differentiate the process from hot pressing, which is called there simply sintering. This is actually not quite true. ISO defines sintering as a thermal treatment of a powder or compact at a temperature below the melting point of the main constituent, for the purpose of increasing its strength by bonding together of the particles. Hot pressing however is a special case of sintering where additional to temperature external pressure is applied. In the following, the major differences are shown as well as the state of the art. 2 The Theory of Sintering Sintering is a diffusion driven process, wherein diffusion is a macroscopically material transportation in solid, liquid or gaseous materials. Fick [1] describes it mathematically in his laws and Jost [2] continues that diffusion depends on temperature as well as on the material. A good estimation is that the diffusion D is proportional to the melting point of a material. The laws from Fick and Jost allow the following interpretation: D Surface > D Phase Boundaries > D Grain Boundaries > D Volume D Surface : D Phase Boundaries : D Grain Boundaries : D Volume : Fine powders are sintering faster than coarse powders due to the increased particle contacts. For the same reason cold pressed compacts are sintering faster than loose powder. Elemental powder sinters faster than prealloyed powder. Powder with fine micro structure increases grain growth. Powder with high dislocation density sinters faster. In the diamond tool industry hot pressing can not be fully avoided until today. There are simple explanations for this: The sintering process slows down after closing of the open porosity, external pressure however, as for the hot pressing, maintains a consistent driving force [3]. The principle is called diffusion induced plastically flow process, also called as Nabarro Harring creep. German [4] defines the Nabarro Herring creep as a time dependant plastic deformation that occurs below the yield strength of a material due to the combined effects of temperature induced atomic diffusion and stress. In practical application 1

2 of the hot pressing process it means that the sintering process runs faster at lower temperature by achieving a higher density. Keeping in mind that diamond starts degrading above 600 C, a low sintering temperature can be a quality enhancement feature for diamond tools. Figure 1: Thermal damages of diamond Figure 1 shows the main thermal damage mechanisms of diamonds: Oxidation, micro cracks and graphitization. Oxidation comes from surface oxygen of the diamond surrounding powder particles. It appears as a roughened surface of the diamond crystals because the oxygen removes graphite from the diamond surface according to the Boudouard equilibrium by a CO/CO 2 reaction. Mechanical cracks start from metallic inclusions inside of the diamond crystals. The different thermal expansion of metal and diamond forces internal tension and finally internal micro cracks. Graphitization comes from solution and reprecipitation processes of Carbon. Metals which have a solution potential for carbon are dissolving Carbon (Diamond) in their lattice during heating. During the cooling cycle the solution potential decreases and hexagonal Carbon (Graphite) will be reprecipitated. It appears as a black layer around the diamonds. 3 Pressureless (Free) Sintering of Diamond Tools Why taking pressureless sintering into account, if hot pressing has so obvious advantages? Most important are certainly economic reasons. Despite of increasing raw material cost the prices for diamond tools are still dropping. Due to competition pressure, rising manufacturing cost are not passed on to the consumers, therefore margins are falling. More economical production processes are an effective method to decrease cost. Technical progress is another reason. The quality of synthetic diamonds has improved continuously during the last 25 years. The level of metallic inclusions in high quality synthetic diamonds is nowadays so low, that the risk of internal micro cracks has drastically dropped. Another important aspect is the bond development. New powders and intelligent powder blends are able to compensate substantially the disadvantages of free sintering. This is only possible if the process is well understood and some important conditions are met. 3.1 Cold Pressing and Shrinkage Contrast to hot pressing the quality of cold pressing is one of the most important processing steps. Hot pressing is more forgiving since cracks and uneven density distribution may heal during the process. This is not the case for the classic sintering process. Deficiencies from cold pressing will have a dramatic impact, especially uneven green density distribution leads to dimensional distortion. Many diamond tool producers are still working with old and mostly 2

3 gravimetric working cold presses, where the double action principle is not or only partly realized. Those presses are not suitable for the free sintering process. Double Action Single Action Figure 2: Position of the neutral zone during cold pressing The area of low green density, which is caused by friction between powder particles itself and by friction between powder particles and the wall of the die, is called neutral zone. The position of the neutral zone has to be in any case in the center of the green compact. Figure 2 shows how the position of the neutral zone is influenced by cold pressing. The green density, which can be achieved by a given pressure, depends on the mentioned friction and on the used type of powder. Appropriate measures are able to influence the green density, but not the position of the neutral zone. During cold pressing a higher pressure causes attaching of powder particles to each other, which increases the green density. At a further increase of pressure the particles become plastically deformed. Thus attaches the particles to each other even closer and the density continues to increase. This effect can be reached at a lower pressure as well by different powder properties or by addition of lubricants. Figure 3: Impact of green pressure on green density [5] Figure 3 shows the green density as a function of pressure. At a given pressure a higher green density can be achieved by annealing the Iron powder, which comes from easier plastic deformation. More effective however is the addition of lubricants. Zinc stearate ore even better modern micro waxes, which are reducing the inner friction, are more effective. Influencing the green density has an eminent meaning in classic free sintering. It defines the shrinkage during the sintering process significantly. Whereas the size calculation for hot pressed bodies is just a function of the linear shrinkage, near net shape sintering without pressure becomes much more demanding. Size calculation in classic sintering requires 3

4 much more attention because the body shrinks in three dimensions. With free sintering the shrinkage depends on: Powder behavior, size of the green body and green density. Since powder behavior is a material property its selection is a matter of bond design. The size of the green bodies and the green density will be influenced from the cold compaction process. Two of the three dimensions are defined by the opening size of the compaction die; the third dimension has to be adjusted by the cold pressure. Figure 4 demonstrates the difference in shrinkage between hot pressing and sintering. The opening size of the cold pressing die is 40 x 5 mm. The volume after sintering by achieving full density is calculated to 200 mm³. Two green bodies are pressed, one is pressed to a height of 20 mm, and another one is pressed to a height of 15 mm (a). By hot pressing the body sinters to a height of 10 mm (b). Due to the differences in green density of the two cold pressed bodies, the shrinkage after sintering is different (c), although the volume of (b) and (c) is equal. Figure 4 shows the differences in shrinkage in more detail in which the linear shrinkage S per axis is calculated by the following equation: S V V 2 = 1 3 (1) 1 Figure 4: Difference between one- and three-dimensional shrinkage Figure 5: Three dimensional shrinkage per axis 4

5 As you see depends the absolute shrinkage on the green density. According to the example from figure 4 is the shrinkage therefore much bigger in length than in width. The green density is so far beside the green geometry an important tool for adjusting the final size. This becomes more evident in figure 5. To achieve the same final size after sintering as to hot pressing a different tool for cold pressing is required and the final size adjustment comes from green density. 3.2 Sintering As to the different sintering mechanisms free sintering requires other bond systems than hot pressing. An example will illustrate this (figure 6). Cobalt powder is still one of the most important raw materials for the production of diamond tools. Using the quality extra fine (1.4µm FSSS) a hardness of 300 HV10 can be achieved at temperatures between 750 C and 800 C. The sintering time takes about 3 5 minutes and the theoretical density becomes % (blue curve). Free sintering shows a different situation. At free sintering (green curve) the maximum hardness is achieved at about 950 C after a sintering time of one hour. The hardness does not exceed more than 225 HV10. Figure 6: Hardness of Cobalt With every sintering an optimum between elimination of porosity and avoidance of grain growth should be found because grain growth is the reason for hardness decrease at higher sintering temperature. By reducing the number of grain boundaries the material decreases its inner energy. This is documented in grain growth and associated lower strength and thus lower hardness. Figure 7 shows the formation of grain growth at two sintered Cobalt samples. Figure 7: Grain growth at sintered Cobalt (1.4µm FSSS) Left image: No grain growth. Right image: Grain growth 5

6 As to be seen in figure 6 free sintering require clearly higher sintering temperatures. This is a major handicap. Lower Hardness means lower strength, lower strength means lower diamond retention of the bond. Avoidance of grain growth is of crucial meaning for the bond development for free sintering of diamond tools. This can be achieved by different methods. As described above the diffusion can be forced whereby the sintering temperature can be lowered. Fine powders are in any case indispensable. Furthermore reaction sintering is another important measure. Reaction sintering means among other the sintering of elemental powders so that alloying takes place during the sintering process. The advantage is that with the disappearance of the primary phases and phase transformation a grain refinement is accompanied. In parallel micro alloying limits the grain growth. By carrying out so called doping smallest quantities of defined phases are attached to the grain boundaries. This leads to sustain suppressed grain growth without or with minimum changes of the material properties (figure 6, red curve). 3.3 Sintering Furnaces and Sinter Atmospheres The selection of the furnace is for capacity aspects without ignoring the flexibility which is an important factor in diamond tool industry. Two types of furnaces would be the first choice: The muffle furnace on the one side and the belt or push-through furnace on the side. The muffle furnace is a batch furnace. It is filled, closed and then follows a preset time temperature profile. With the belt furnace, it behaves differently. It is very long and has fixed heating zones. The sintering material goes through the heating zones on a belt; the time temperature profile is adjusted by the speed of the belt and the temperatures of the various zones. Both furnaces have about the same capacity. The belt furnace has the far higher flexibility, because the belt speed and the temperature can be changed at any time. With the muffle furnace the program sequence must be awaited. Beside the furnace selection the choice of the atmosphere plays an important role. Figure 8 gives assistance for the proper decision. To achieve complete densification the vacuum is indispensable otherwise smallest gas inclusions remain. For the sintering of diamond tools this fact is minor important, as the technical effort is relatively high and expensive. A pure nitrogen atmosphere is only for safety aspects of interest. Pure hydrogen is expensive and can be substituted by mixtures with hydrogen and nitrogen. Figure 9: Impact of the atmosphere on sintering During sintering hydrogen reacts with oxygen to form water in the form of steam. Therefore it is from high importance to look for sufficient gas flow to carry away the steam. Otherwise steam leads again to oxidation during cooling. From this point of view the water steam content of the protective atmosphere need to be respected. The steam content in the gas is measured as dew point. The dew point is defined as the temperature where water steam starts to condense [6]. A very good dew point of about -40 C means that the steam content is less than 0.02% (figure 9). 6

7 Figure 9: Steam content on volume percent vs. dew point [7] 4 Economy and Cost Saving Potential The production cost of a diamond segment can be divided in three blocks: Production cost, powder cost and diamond grit cost. Figure 10 shows three examples for different segment geometries. As a segment producer has limited influence on the raw material cost, he has dominant influence on his production cost. This block is about 30% to 60% of the whole production cost. This is exactly the area producers can attack by using free sintering. Figure 10: Production cost of selected segments In figure 11 the capacity of a small muffle furnace has been calculated to about 3500 segments per day, while the capacity of a belt furnace is about 4000 segments. A common hot press sinters about 50 segments per cycle, which takes roughly 20 minutes. This leads to a daily capacity of 1200 segments. Cost for graphite and packing are eliminated. Filling of the furnace is easier and faster. Further cost savings can be realized if the cold press is equipped with a handling system, which can pack the segments directly on suitable charging plates. Another advantage is, that segments are leaving the furnaces metallic shiny without a sinter skin. 7

8 Figure 11: Capacity estimation, segment size: 40 x 10mm 5 Summery Free sintering is an efficient possibility to lower the production cost. Modern bonds and a good understanding of the process and the process parameters are allowing manufacturing high quality diamond segments, which can achieve a comparable performance than hot pressed segments. Even for smaller series the belt furnace allows sufficient flexibility. Free sintering might not be the solution for all diamond tool applications but a bigger part of common segments can be produced with this cost efficient process. 6 Literature [1] A. Fick, Phil. Mag. (1855), 10, 30. [2] W. Jost: Diffusion in solids, liquids and gases. Academic Press Inc., New York, 1960 [3] Salmang Scholze, Keramik (2007), 7th edition. [4] R. M. German: A Z of Powder Metallurgy, Elsevier Ltd, [5] F. Eisenkolb, F. Thümmler: Fortschritte der Pulvermetallurgie. Bd. 1, Grundlagen der Pulvermetallurgie, Berlin, Akademie-Verlag 1963 [6] P. C. Angelo, R. Subramanian: Powder Metallurgy: Science, Technology and Application, PHI Learning Private Ltd, New Delhi, 2008 [7] G. S. Upadhyaya: Powder Metallurgy Technology, Cambridge International Science publishing,