A novel look at the metal matrix used in diamond impregnated tools for cutting stones
9 May 2011
By Andrzej Romanski and Janusz Konstanty, AGH-University of Science and Technology,Faculty of Metal Engineering and Industrial Computer Science, Mickiewicz Ave 30, 30059 Krakow,Poland
The world market for diamond impregnated tools has shown significant growth over the last decade. The main reason for this growth is the higher performance of the diamond tools, including tool life and productivity, when compared to other tool grades. It is forecast that by 2013, the global market value for diamond tools will reach over USD 7,000 million. Diamond tools can be found throughout industry, but especially in stone and construction and general machinery, which between them share about 45% and 27% respectively, of the global market for diamond tools. The types of diamond tools in highest demand are saw blades (27%), wire saws (20%) and grinding wheels (19%) .
Typical techniques used to manufacture diamond tools include hot pressing, sintering followed by cold pressing and hot isostatic pressing. All these manufacture routes require powders of material being used as a metallic matrix and diamond grits. Over the last decade, the cost structure of diamond tools manufacture (cost of raw materials, tools as moulds, used in production, etc.) has changed dramatically. In the early 1990s, the total cost of a diamond tool was mainly dependent on the price of diamond. The diamond cost contribution to the total cost of the tool was over five times higher than cost of the other components. Also prices of metallic powders, mainly cobalt powders, as well as graphite moulds and electrodes used in hot pressing, played a major role in the total cost structure. Today the structure of the total diamond tool cost has markedly changed thanks to the availability of synthetic diamond from China on a wider commercial scale, thesystematic growth of production, coupled with the use of hot presses equipped with gas chambers (to protect the graphite mould against oxidation and in doing so extends the mould’s life). Table 1 shows the comparison of a total cost of a segment 40x10x6 mm used in a 800 mm circular saw for cutting granite.
Due to a unique combination of high strength, toughness and adjustable resistance to wear, as well as excellent retention for diamond, cobalt and cobalt-base matrices impart long life and excellent cutting performance to the tool. That’s why for many years, in most applications, cobalt and its alloys powders have been found to be the best material used as a matrix in diamond impregnated tools. But the unstable price of the raw material – cobalt electrodes are used in the powders’ production (Fig.1), has forced diamond tool manufacturers to seek alternative solutions: to substitute cobalt powders, even partially, by other, cheaper materials.
As a consequence of these investigations, a few new pre-alloyed powders based on iron and copper have been developed and introduced to production at a commercial scale. The best examples of such powders are Next, Keen by Eurotungstene (France) and Cobalite by Umicore (Belgium) – Table 2 [4-8].
The ability to better engineer metal matrix properties depends on understanding the events taking place at diamond-metal matrix interface. Irrespective of cutting mode, the diamond grits are subjected to normal and tangential forces which are trying to pull the diamond grits out of the matrix. During sawing operations the matrix adjacent to the working diamonds is periodically loaded and, if merely subjected to elastic deformation, it reverts to its original shape. Thus it is postulated that the potential capacity for diamond retention of the matrix should be associated with its Young’s modulus and offset yield strength as these properties govern the amount of energy that must be spent to induce plastic deformation of the material. Hypothetically, a combination of low yield stress and high Young’s modulus will facilitate matrix deformation and lead to debonding between the diamond and the matrix. After hot pressing of mixture containing metal powder(s) and diamond grits, during cooling to room temperature, diamond particles are tightened by metal matrix. This is the effect of mismatch between thermal expansion coefficients: it can be assumed that thermal expansion coefficient for metal matrix is about 10 times greater than for diamond. Thus the stress and strain fields occur at particles’ surroundings. By applying computer modelling, it is possible to evaluate the generated stresses and strains depending on input data related to matrix properties and diamond-matrix interface [13-18]. It is expected that the results obtained should enable to show a correlation between the matrix’s mechanical properties and its retentive capability for diamond grits. The simulations were performed on an elastic-plastic 3-D model, where the linear tetrahedral elements type C3D4 and linear hexahedral elements type C3D8R were used to mesh a matrix and diamond particle, respectively. The mesh configuration of the model is presented in Fig. 2, whereas the mechanical and thermal properties of the hypothetical matrices are summarised in Table 3.
To simplify the calculations, it has been assumed that the matrix is an ideally plastic material which retains its room temperature properties up to the hot pressing temperature. Also, it has been assumed that the hypothetical matrix expands thermally in a similar manner to cobalt, widely used as a matrix in the diamond tools industry. It is suggested that the potential retentive properties of the metallic bond can be associated with the amount of energy of its local, near the diamond particle, elastic and plastic deformation. It is also postulated that the higher stress generated in diamond grit at the room temperature, the better the bonding is between the particle and metal matrix, and, in consequence, the better is the diamond retention capacity of the bond. With these assumptions, the potential retention properties of the hypothetical matrix were estimated by calculation of following energies:
• total strain energy ALLIE of the matrix – a sum of elastic and plastic deformation energies,
• energy dissipated by plastic deformation of the matrix – ALLPD,
• recoverable strain energy accumulated in the diamond grit (RSE).
All investigated parameters were calculated for each combination of mechanical properties of the matrix and friction coefficients. Examples of maps showing distribution of stress/strain generated around diamond crystals are presented in Figs 3 and 4. The calculated values of ALLIE, ALLPD and RSE are summarised in Tables 4-6 and graphically analysed in Figs 5-7.
As shown in Figs 3 and 4, the local stress is proportional to the yield strength of the matrix, whereas the size of plastically deformed zone shows the inverse trend.
From Table 4 it is evident that ALLIE markedly increases with the yield strength and Young’s modulus while the effect of friction can be neglected. The effect of yield strength on ALLIE is more pronounced for lower diamond protrusion and the opposite situation is seen for Young’s modulus. Interestingly, the effect of the coefficient of friction increases by a factor of ~2.5 as the height of diamond protrusion rises from 25 to 100m.
Contrary to ALLIE, ALLPD is mainly affected by Young’s modulus, and its dependence on yield strength is markedly weaker (Table 5).
It is noteworthy that RSE strongly depends on the yield strength of the matrix whereas the other two factors are of secondary importance (Table 6).
All in all, the generated data indicate that ALLPD and RSE are mainly influenced by Young’s modulus and yield strength, respectively, whereas both these properties equally affect ALLIE. The variation of friction coefficient between 0÷1 has negligible effect on the calculated energies, especially for low diamond protrusion. It should be noted, however, that coated diamonds are chemically bonded to the matrix and the friction coefficient may attain values markedly higher than analyzed in this work. In such a case the role of friction must not be ignored.
The calculated energies apparently depend on the height of diamond protrusion. It seems reasonable to assume that the working height of diamond protrusion is above 100m  and, therefore, the results obtained for this value suggest that the factors which contribute to improved diamond retention can be ranked in order of decreasing importance as follows:
1. yield strength of the matrix – which markedly affects RSE thus enhancing the hold on unloaded diamond crystals
2. Young’s modulus of the matrix – which markedly affects ALLPD and may potentially constrain plastic deformation of the matrix around loaded diamonds
3. friction coefficient at the matrix/diamond interface – which has a minor effect on the calculated energies within the range from =0 to 1.
The results obtained indicate that in order to improve the retention capabilities of the matrix it is necessary to maintain both elastic and plastic deformation around the diamond grit. After hot pressing, upon cooling to room temperature, stress/strain fields are generated around each diamond particle. But during machining, the matrix wears and diamonds emerge constantly over the surface. Due to matrix removal, the stress/strain fields surrounding the diamond decrease, and diamond grits are more weakly held by the matrix in consequence. Thus in order to maintain a strong bonding between the diamond grit and improve the tool’s productivity, new kinds of powders are needed that would ensure a increase in stress/strain during the machining process.
At the Faculty of Metals Engineering & Industrial Computer Science of AGH-University of Science and Technology, Krakow, Poland, intensive research has been undertaken to invent new powders which would be characterised by increased diamond retention. The investigations are focused on commercially available iron powders, which are alloyed by selected alloying elements, after hot pressing upon cooling to room temperature, give a metastable austenitic microstructure. The idea to increase diamond retentions lies in martensitic transformation induced both by shear stresses and plastic deformation which takes place during stone machining. Due to the higher specific volume of martensite compared to austenite, additional stresses are generated in the matrix surrounding the diamond grit and it is this that intensifies the diamond-matrix bonding.
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