New Materials and Processes
Iron Powder-based Metal Matrix for Diamond Cutting Tools: A Review
2020 Volume 60 Issue 8 Pages 1571-1576
Details
2020 Volume 60 Issue 8 Pages 1571-1576
New material technology, advancement in manufacturing technology and development of alternate materials are key enablers for today’s process and product development. Although Diamond cutting tools has a long history started back in 1862, this industry has gone through a tremendous change in last 50 years with the development of synthetic diamond and continues exploration of alternate metal matrix materials (alternate to cobalt). For each specific application, powder for metal matrix needs careful selection based on its chemical composition, size, shape and thermal stability. Cobalt powder was the most commonly used metal matrix powder. However, fluctuating price and health concerns associated with cobalt powder has compelled the industry to search for more economical and sustainable alternative. In this context Iron and Iron based alloy powders turned out to be promising metal matrix powder alternatives for use in diamond cutting tools catering cutting, drilling, grinding, etc applications.
This paper gives an overview on the diamond cutting tools history, evolution, trends and developments with special focus on the usages of iron powder as an alternate matrix material for diamond cutting tools.
Diamond as a material exhibits many superior properties among which, high hardness, very low thermal expansion and cutting edges enables them to act as a good constituent in cutting tools.1) Mechanical superiority of diamond over other hard cut material is shown in the Fig. 1.2) Diamond cutting tools are diamond impregnated metal matrix, whose embedded diamonds are responsible for cutting hard to cut materials. Modern diamond cutting tools are applicable in stone cutting, construction, road repair, wood working, production of various parts made of glass, ceramics, metals and plastic.3)
Mechanical properties of different hard material.
A typical diamond cutting tool manufacturing process involves, a series of unit operations and processes as shown in Fig. 2. Firstly, selected metal matrix powders and diamond grits are proportionately mixed and are subjected to hot compaction. This hot compacted part is known as segments. Segments are further brazed at steel core to form a typical diamond tool as shown in Fig. 3. The grade of steel cores materials depends upon whether the segments are brazed, or laser welded.4) For effective usage of diamond tools, an ideal balance between the tool life and cutting rate is important and is achieved by choice of metal matrix. Metallic matrix plays two basic functions, holds the diamond tight to avoid any intermittent pull-out and ensures wear out at a compatible rate with the diamond breakdown.3) If matrix is too hard and wear out is slow, this can delay the exposure of new diamond cutting edges and make diamond grits dull.5)
Manufacturing process of diamond cutting tools. (Online version in color.)
Diamond cutting tools (circular saw blade). (Online version in color.)
In most of the commercial diamond cutting tools, the matrix used is a metal alloy. In terms of volume of production, metal-bonded tools account for around two-thirds of the whole bonded-grit diamond tool market.6)
No standard literature is present to acknowledge the research work happening in and out the domain of metal matrix relating to diamond cutting tools. In this review, authors have attempted to elucidate ongoing research and new trends in metal matrix powders. More focus is laid on the modern alternative “iron powder” and its usage in diamond cutting tools.
In 1862, Swiss engineer J. R. Leschot has proposed the idea of making diamond drill bits which soon found practical application. The first diamond circular saw blades were developed by Felix Fromholt in France in 1885.7) Subsequently, the developments in the field of powder metallurgy led the way to the development of diamond grit impregnated saw blades, which were put into operation around 1940. In the past 50 years, notable developments in this field took place and one such prominent is the invention of synthetic diamonds. With a 50-fold increase in its industrial consumption, at present day, these synthetic diamond grits account for around 97% of all industrial diamonds consumed.6,7) Chronological developments in the metal bonded diamond cutting tools segment is showcased in the Fig. 4.6)
Chronological listing of development for diamond cutting tools. (Online version in color.)
The cutting ability of diamond tools and tool life are dependent evidently on the mechanical, chemical, and tribological properties of the metal matrix.8) Selection of right metal matrix plays a crucial role to achieve the optimal properties. Chemical composition, particle shape, particle size distribution and amount of impurities of metal powders are taken into account while choosing a right metal matrix.2) Particle shape and particle size distribution affect the flowability and sintering behaviour of matrix powder. In case of large particle size (> 45 μm), most of the densification is achieved in compaction whereas for fine powder, densification takes place in sintering process. Table 1 provides a behaviour contrast for flowability and sintering nature of two grade of iron powder.9) Finer powder result in smaller grain size and hence higher matrix hardness. Carbonyl iron powder with finer size gave higher hardness. Finer size powder offers high surface area that leads to high sintering activities at low temperature (800–900°C) and this help in saving energy cost in production.
| Carbonyl iron powder | Water atomized iron powder | |
|---|---|---|
| Size (μm) | 5 | 75 |
| Shape | Spherical | Irregular |
| Alloying element | 2% Ni and 0.5%C | 2% Ni and 0.5%C |
| Flow time (Second) | Non-Flowing | 26 |
| Shaping process | Injection molding | Die compaction |
| Green density | 4.7 g/cc | 7.2 g/cc |
| Sintering temperature and time | 1300°C for 60 min | 1150°C for 30 min |
| Final density | 7.8 g/cc | 7.2 g/cc |
| Hardness | 91 HRB | 69 HRB |
Presence of impurities also hampers the metal matrix properties. It is reported that Sulphur imparts brittleness to cobalt matrix even when present in trace amounts of 50–60 ppm.6) Oxides remaining in the metal powder prohibit grain growth which help in high hardness in matrix, but with increasing oxygen content, the impact toughness of matrix drops. High oxygen content hinders the process of neck formation between powder particles during sintering and results in poor sintering and lower impact toughness. It can be seen in Table 2, Impact toughness of the cobalt matrix drops dramatically by increasing oxygen impurities.10)
| Oxygen content (wt.%) | Impact toughness (J/cm2) |
|---|---|
| 0.007 | 19 |
| 0.091 | 14.9 |
| 0.282 | 7.8 |
| 0.444 | 5.9 |
Development of metallic matrix pursues three key issues, optimal balance between the abrasion of the matrix and the wear of the diamonds, high sinterability of matrix to minimize the risk of graphitization during the production and good wettability between diamonds and matrix to get a chemical as well as a mechanical bond.11) In particular Co, Cu and Fe are the major metal powders used for preparing metal matrix for diamond cutting tools.
The graphitization is a critical issue for the use of synthetic diamonds in the industry, in this process synthetic diamond loses strength after reacting with oxygen. Temperature and oxygen that cover the diamond surface affect the graphitization process. Diamond start losing strength from about 800°C temperatures and rapid graphitization occur above 1000°C. Metallic binders that possess high affinity with carbon, like Co, Fe, Ni accelerate the graphitization process.12)
Wetting of diamond surface with metal matrix is governed by the nature of their bonding with diamond. Bonding mechanism between diamond and metal matrix may be either mechanical or chemical or a combination of both. Because of the high strength interatomic covalent bonds in diamonds, chemical bonding at the interface is only possible if the metal matrix possesses high chemical affinity to carbon atoms.10) The bonding strength between matrix and diamond is determined by the adhesion energy of the interface that is closely connected with the wetting behavior of the metal matrix on diamond or carbon surface. Metals such as Cu, Ag and Au have wetting angle with diamond in the range of 120–150°.13) This shows poor wettability and the bonding between matrix and diamond is mechanical in nature.
In cutting operation, the bond strength at the interface between the diamond and the metal matrix must withstand the stresses developed at the individual diamond to avoid any intermittent pull-out of diamond from matrix. The bonding strength between matrix and diamond is determined by the adhesion energy of the interface that is closely connected with the wetting behavior of the metal matrix on diamond or carbon surface. B. Kieback and co-workers also reported the importance of wetting angle and adhesion energy, the wetting angle and adhesion energy of Co, Cu and Fe with diamond and carbon are given in Table 3.10) It is evident from the tabulated report that Fe, Co and Cu possess adhesion energy of 3.34 J/m2, 3.2 J/m2 and 0.23 J/m2 respectively. Fe possess high adhesion energy because of high affinity of Iron with Carbon.10) On the other hand, Copper possess very low adhesion energy due to low affinity of copper with carbon. Thus, Fe and Co exhibit superior binding properties as compared to copper. However, many researchers have attempted to improve the wetting behavior by addition of active elements.
| Metal matrix | Temperature (°C) | Wetting angle (degree) | Adhesion energy (J/m2) |
|---|---|---|---|
| Co | 1500 | 48 | 3.20 |
| Cu | 1100 | 145 | 0.23 |
| Fe | 1550 | 37 | 3.34 |
In case of Cu based matrix, by adding 11 wt% Sn and 0.7 wt% Ti wetting angle improve from 145° to 7° with good wettability.10) To modify the adhesive properties of copper and increase the bonding strength of matrix, alloying elements such as Titanium and Tin are added in copper matrix. It is reported by B. Kieback and co-worker, adding 12.8% Ti in copper matrix increases the adhesion energy to 2.66 J/m2.10) Other alloying powder such as Copper, tin, tungsten carbide and bronze are also added in a cobalt-based powder matrix to aid densification and modify the abrasion resistance properties.14,15,16) Cu and Sn enhances the liquid phase sintering, it provides a faster diffusion rate, pore elimination and thereby aid densification.5)
All these properties of metallic powder results in a matrix which gives adequate toughness and resistance to wear after hot compaction.
One another significant factor that influences the selection of a metallic matrix powder is its cost. High production cost of diamond cutting tools is largely attributed to the high price of the powder being used for the matrix.17) Cobalt and copper metal possess cost in the range of 15–30 USD/lb and 2.5–3.5 USD/lb with very high fluctuation in the past decade18) whereas, iron metal in comparison possess very low cost and minimal fluctuation. On the other hand, Co and Cu are also considered to be toxic.19,20)
Thus, considering the powder properties, cost and environmental impact current diamond cutting tool manufacturers are emphasizing more to look for a cheaper, better and prominent powder alternative.21) Selection of suitable powders fulfilling such requirements need to be addressed in a scientific method rather than empirical approach. Finding an environmentally acceptable, economically feasible and equally effective alternative is been a daunting challenge for diamond tool industry. Lot of effort is being put forward into research and development of metal powder matrix that can reduce or substitute or eliminate cobalt powder. One such outcome is the usage of “iron powder”.
Diamond cutting tools manufacturers started developing iron based new matrix powders and alloy. Over the past decade, lot of developments took place to produce fine powder and in the existing scenario only few process like atomization, reduction, carbonyl, and electrolytic synthesis are economically viable and well accepted.22) Shape and Morphology of all the four grades of iron powder are presented in Fig. 5, Carbonyl iron powders are spherical in shape whereas atomized and reduce iron powder are more irregular. Electrolytic process gives dendritic shape iron powder. Atomization production technique includes spraying of molten metals with the help of pressurized fluid or centrifugal force. However Conventional water atomization method shows disadvantages of coarse particle size and high oxygen content in products.23) Bulk of the iron powder produce with this technique targeted to an average particle size of 100 μm.9) Apparent density of powder varies in the range of 2.5 to 3.2 g/cc. These powders are extensively used in Powder metallurgy parts for automobiles, welding rod and filters. It has been reported by Zhi-qiang CHU and co-workers that ultrahigh pressure water atomization (UPWA) process can offer pre-alloyed atomized fine iron powder with average particle size 50 μm that are more suitable in diamond tool application.23) In the case of reduction technique, Iron oxides are reduced using solid or gaseous reducing agent. This technique yields a spongy mass of solid metallic iron which can be comminuted to iron powder. Purity of iron oxide and efficient intermediate processes such as refined by mechanical, magnetic and chemical process are very important to ensure the purity of iron powder in this technique. Average particle size for reduced iron powder lies below 150 μm with apparent density in the range of 1.5–2.9 g/cc. These powders are used in low end powder metallurgy parts and body warmer application. In carbonyl process, powder is obtained from metal compound. This manufacturing technique gives very fine, highly pure and very active iron powder. Iron powder purity of 99.5% can achieved in this technique. Average particle size for Carbonyl iron powder lies in the range of 1.5 to 8 μm with apparent density in the range of 2–3 g/cc.24) These powders are used in different application area such as Diamond cutting tools, Binder in hard metal alloy, Metal injection molding and Electronic application.9) Electrolytic method for iron powder production involves cathodic deposition of fine iron powder, where electrical current act as reducing agent.25) In this process reduction takes place at the cathode and oxidation takes place at the anode in presence of suitable electrolyte. Average powder particle size in this process are 100 μm that can be even grinded in finer size below 20 μm for specific application. A wide range of apparent density 0.4 to 4 g/cc can be achieved in this process. These powders are used in Diamond cutting tools, food fortification, chemical reagent and Metal injection molding.
Typical morphology of different types of iron powder.
Aforementioned grades such as Electrolytic iron powder, carbonyl iron powder, atomized and reduced iron powder are used in Diamond cutting tools metallic matrix.1,23,26) In diamond cutting tools application, fine (below 45 micron) iron powder are preferred. Lot of efforts are also going into Research and development to improve the properties further in terms of preventing diamond graphitization by coating the diamond grit and increasing the mechanical properties in Iron based matrix powder. Using relatively less costly water-atomized powders, development of pre-alloyed powders and the introduction of the so-called “free” or pressure less sintering are challenges still requiring considerable R&D efforts.3) Information regarding the concentrated effort to develop and utilization of iron powder-based matrix from technical point of view is tabulated in Table 4.
| Sr No | Author & year | Matrix composition | Purpose of study | Ref |
|---|---|---|---|---|
| 1 | J. Borowiecka- Jamrozek (2016) | 45% Fe+50% Cu+3% Zn+2% Sn. | Study of Hot consolidation of iron-based matrix and evaluation of its diamond particle retention properties | 17) |
| 2 | J. S. Konstanty (2013) | 85% Fe+15% Cu & 80.5%Fe+16.5%Cu+3% Sn | Evaluation of wear resistance of Fe based matrix with different microstructural phases | 16) |
| 3 | Luciano Jose de Oliveira (2015) | 100% Fe | Effect of Iron on the graphitization of diamond | 12) |
| 4 | L. J. de Oliveira (2007) | Fe+ (5,10,15,20)%Cu+1% SiC | Optimization of alloying element with Fe matrix and its wear properties | 29) |
| 5 | Chris Peters (1997) | Fe based matrix | Comparison of diamond tools performance made of iron based and cobalt based matrix | 33) |
| 6 | A. P. Barbosa (2008) | Fe+15%wtCu+(0,10,20,30)%wt Co | Effect of alloying element Co on hardness and wear resistance of Fe–Cu matrix | 19) |
| 7 | Janusz Konstanty (2006) | (66,70,72,80)%Fe+(3,7,20)%Cu+(10,25,27)% Co | Matrix material selection, diamond tool Design and manufacturing process | 3) |
| 8 | K. An (2013) | 28% Fe+30%Cu+35% Co+ 5% Sn+2%Ni | Study of sintering behavior of alloy to optimize the sinter density | 5) |
| 9 | P. A. Loginov (2019) | (Fe+15%Co+10%Ni)+(0.5–3%Ti or 0.5–4% TiH2) | Improvement in mechanical properties of matrix by doping of Ti and TiH2 | 13) |
| 10 | Yu-Zan Hsieh (2001) | Fe+11.3%Ni Vs (Cu+26%Co+6%Sn+3%Ni) | Comparative study of Cu vs Fe based metals matrix in Pressure less-sintering condition | 26) |
| 11 | W. Tillmann (2010) | 60%Fe+40%C | Advantage of Spark Plasma Consolidation and Spark Plasma sintering process over conventional sintering in in case of high Iron containing metal matrix | 34) |
| 12 | Xiaojun Zhao (2019) | 48.89% Fe+39.87% Cu+4.94% Sn | Effect of sintering temperature on Fe-based pre-alloyed powder on the microstructure and holding strength of impregnated diamond bit matrix | 28) |
| 13 | Meng Li (2016) | Fe+10%Ni with/without (0.25–1.5)%B | Effect of Boron addition in Fe based matrix in case of Pressure less infiltration sintering | 30) |
| 14 | Joanna Borowiecka-Jamrozek (2017) | (48–70)%Fe+(29–45)%Cu+(1–3)%Sn+(2–4)%Zn+(1–3)%P | Modelling of retention of a diamond particle in metal matrix | 31) |
| 15 | N. B. Dhokey (2013) | 46.6%Fe+13.5%Cu+20%Co+10%FTC+6%Al+1%Mg | Study the effect of velocity and load on the wear rate of metal matrix | 32) |
| 16 | Zhi-qiang CHU (2016) | 55%Fe+ 30% Cu+ 5.8%Sn+5.7Zn | Pre-alloyed powders for diamond tools by ultrahigh pressure water atomization process and evaluation of its mechanical properties after sintering | 23) |
High sintering activities in iron-based matrix at 800–880°C has been reported by J. Borowiecka Jamrozek and co-workers. Sintering density > 97% of theoretical density is achieved in this study.8) This study shows that iron-based matrix offers good diamond particle retention properties. Adhesion of diamond particle into bonding matrix is important because if there is poor bonding between matrix and diamond adhesion, premature loss of the diamond grit takes place.27) To achieve high adhesion between diamond and Fe matrix, coating of diamond particles with TiC has been reported by Luciano Jose de Oliveira and co-workers.12) The chemical adhesion between TiC and Fe matrix is due to the formation of Fe-Ti solid solution during hot pressing. Roughness of coated diamond grits also improve the mechanical adhesion as compared to smooth surface of uncoated diamond. An improvement of 8–10% in yield strength and rupture stress in matrix have been reported in this study. Coating layer of TiC on diamond grits helps in avoiding the direct contact of diamond with Fe, this leads to preventing the graphitization of diamond. Investigation being carried out by P. A. Loginov and co-worker reports that improvement in mechanical properties and adhesion to diamond can be achieved by doping titanium hydride (TiH2) into iron-based matrix. Alloy composition of Fe+15 wt%Co+10 wt%Ni was doped with 0.5–4 wt% TiH2. Study revealed that at 3 wt% TiH2, bending strength of matrix increased from 1980 MPa to 2921 MPa. The significant increase in bending strength of matrix can be due to the formation of Fe–Ti phase.13) Titanium interact with carbon at the temperature range of 550–950°C and form carbide sublayer. These sublayers reduce the difference in the coefficient of thermal expansion between diamond and matrix and improve wettability at the interface that leads to good adhesion. This study showed that 50 nm thick layer of TiC formed on the diamond matrix interface and 15% diamond surface adhered with metal matrix. According to Xiaojun Zhao and co-worker study, result shows that in case of pre-alloyed iron powder matrix, relative density, bending strength and the plasticity of the matrix increases with increasing sintering temperature till 980°C and after this temperature it get stabilize or decrease slightly.28)
Janusz Stefan Konstanty and co-author work revealed the effect of microstructure on the wear behaviour of metal matrix. Eleven different alloy combination of metal matrix was hot sintered at 850–900°C under 35 MPa pressure. Alloys were designed to have αFe or combination of (αFe + γFe) phases. This study says that inverse relationship between the matrix resistance to 3-body abrasion and the wear rate of diamond-impregnated composites has been found for iron- based alloys containing metastable austenite.16) A. P. Barbosa et al., revealed, the effect of Co addition in iron-based matrix, Fe-15 wt%Cu-(0-10-20-30) wt% Co were blended to study the properties of matrix. Addition of Co promote the formation of Fe–Co solid solution precipitates in Fe matrix which strengthened the matrix. Study shows that 30 wt% Co addition increases the Brinell hardness value from 40 HB to 57 HB and the wear resistant 15.5 time superior than the sample without Co content.19) L. J. de Oliveira and co-worker work revealed the influence of 1 wt% SiC and different composition of copper on wear properties of iron-based matrix. Alloy composition of Fe+(5,10,15,20)wt%Cu+1 wt%SiC were considered in this study. Free sintering of cold pressed compact was done at 1050°C and 1150°C for 25 mins. SiC has density ~3.2 g/cc which is around 2.5 times lighter than iron and copper. At sintering temperature of 1150°C when copper melt and Form the liquid phase, SiC particles emerge in the liquid phase and precipitate on the grain boundary of the iron, this phenomenon strengthens the metal matrix. Higher content of copper in iron also helps in improved adherence with diamond particles. This study shows that the combined effect of high copper content and SiC, alloy composition Fe+20 wt%Cu+1 wt%SiC sintered at 1150°C offer abrasion wear resistance 2 times higher than the other compositions.29) Yu-Zan Hsieh et al., revealed, effect of coating of diamond grit with active element such as Titanium in iron-based matrix. This form a transitional layer between diamond grit and iron matrix and prevent the degradation of diamond. Alloy composition of Fe+11.3 Wt% Ni with coated and uncoated diamond grits were used for comparative study. For the Tool composed of titanium coated diamond grits, the diamond grits with more than one-half of their height protruding out of the matrix were not pulled out of the matrix. This observation is due to the enhanced cohesion between diamond grits and matrix.26) Meng Li and co-worker stated that addition of Boron, helps in reducing the diamond degradation during hot consolidation.30) Boron more easily react with oxygen than carbon. Boron oxide has good oxidation protection and wettability on diamond surface. This help in reducing the diamond graphitization.30)
(1) Metallic matrix in diamond cutting tools is the primary basis on which the tool is developed, and various factors like, tool life, efficiency, wear rate are dependent on it.
(2) Metal powder such as Co, Cu and Fe are the majorly used in synthesis of metal matrix.
(3) Fe being cognate to Co with similar crystal structures and properties, makes Fe a potential candidate to reduce and replace the cobalt consumption in diamond cutting tools manufacturing.
(4) Iron and Iron based alloyed powders have a good economic value and broad application outlook.
(5) Current day research in iron powder metal matrix is focused on improvement in matrix wear properties to overcome the problems being faced by diamond tool manufacturers.
(6) Different combination of alloying with copper, cobalt, Tin and other elements are also being tried to achieve the most optimal properties in terms of uniform microstructure and diamond holding strength in matrix.
