Effect of Sintering Temperature and Boron Carbide Content on the Wear Behavior of Hot Pressed Diamond Cutting Segments

The aim of this study was to investigate the effect of sintering temperature and boron carbide content on wear behavior of diamond cutting segments. For this purpose, the segments contained 2, 5 and 10 wt.% B4C were prepared by hot pressing process carried out under a pressure of 35 MPa, at 600, 650 and 700 °C for 3 minutes. The transverse rupture strength (TRS) of the segments was assessed using a three-point bending test. Ankara andesite stone was cut to examine the wear behavior of segments with boron carbide. Microstructure, surfaces of wear and fracture of segments were determined by scanning electron microscopy (SEM-EDS), and X-ray diffraction (XRD) analysis. As a result, the wear rate decreased significantly in the 0-5 wt.% B4C contents, while it increased in the 5-10 wt.% B4C contents. With increase in sintering temperature, the wear rate decreased due to the hard matrix.


Introduction
Diamond cutting tools are commonly used for cutting, drilling, grinding, and polishing natural stone [1].Diamond cutting tools are comprised of a metallic matrix and cutting grain (generally diamond).The hot pressing method enables the synthetic diamond to bond with the metallic matrix [2][3][4].The two basic functions of the metallic matrix are to hold the diamond tight and to wear at a rate compatible with the diamond loss.Carbides were added to the matrix in order to increase the wear resistance of the metallic matrix.The number of subject-related studies in literature is limited.Meszaros and Vadasdi [5] produced Co-2% WC matrix diamond cutting tools.The study reported that WC controlled the weight loss of the matrix with abrasion and ultimately increased the wear resistance.Oliveira et al [6] used Fe-Cu-SiC powders as a matrix for diamond cutting tools.There was a 14% rate of increase at the hardness level that has a controlling effect on the rate of wear with the addition of SiC.In this investigation, the effect of boron carbide content, known to be the hardest material with the best mechanical properties after diamond and cubic boron nitride [7][8][9][10], and the sintering temperature on the bending strength and wear behavior of diamond cutting segments were studied.

Materials and Experimental Studies
The raw materials used in experiments were bronze powder (85 wt.% Cu + 15 wt.%Sn, purity 99.9%, grain size 45-50 µm), boron carbide powder (79 wt.% B + 21 wt.%C, purity 99.5%, grain size 20 µm), and synthetic diamond grain (grain size 40/50 US mesh).Boron carbide grains were added to the bronze at the amount of 2, 5 and 10 wt.% percents.The amount of diamond was selected as concentrations of 30.Fig. 1 illustrates the SEM micrographs of bronze powder, boron carbide powder, and the diamond grain.As illustrated in the SEM micrographs, bronze powder has an irregular shape, boron carbide powder has a variable structure and is sharp-edged, and diamond grains have a cubic octahedral structure.Bronze, boron carbide powders and diamond grain were homogeneously mixed by the adding paraffin wax in a mixer.The powder mixtures prepared were directly hot pressed in graphite moulds for 3 min at 600, 650 and 700 °C with an applied pressure of 35 MPa on an automatic hot pressing machine.The circular diamond saw blade used in the present tests had a diameter of 300 mm and a steel core of thickness 1.8 mm, 21 pieces of diamond impregnated segments (size 40 x 7 x 3.2 mm) were brazed to the periphery of circular steel core with a standard narrow radial slot.Fig. 2 illustrates the completed circular diamond cutting tool.The relative density, hardness and transverse rupture strength of the segment matrix were determined.The relative densities of segments were measured by Archimedes' principle.Hardness measurements were performed using a Brinell scale with a ball diameter of 2.5 mm and a load of 62.5 kg.The three-point bending tests were performed to determine the transverse rupture strength (TRS) of the segments.The size of the hot-pressed segment for the three-point bending test was 40 x 7 x 3.2 mm.
The wear behaviour of diamond cutting segments was tested by cutting a 500 x 150 x 30 mm andesite using a stone cutting machine.The amount of andesite cut during the cutting process was calculated by multiplying the cut length with the cut depth.The diameters of the circular diamond saw blades were measured before and after the cutting procedure using a digital caliper with a resolution of 10 -2 mm.The wear rate (mm/m 2 ) of each segment was calculated by dividing the radial wear (mm) of each segment with the amount of andesite cut (m 2 ) [11,12].
A scanning electron microscope (SEM) fitted with an energy dispersion X-ray spectroscopy (EDS), an X-ray diffractometer were used to investigate the fractured and wear surfaces, and identify the phase structures, and how the microstructure of segments changed based on the sintering temperature and boron carbide content.

Microstructure
The segments containing B 4 C were successfully produced using the hot pressing method together with a sintering time of three minutes at 600, 650, and 700°C, under a pressure of 35 MPa.Fig. 3 illustrates the XRD pattern of the segments manufactured in the present study.As illustrated, α-Cu, ε-bronze (Cu 3 Sn) and B 4 C phases formed in the microstructure of the segment matrix.The formation of α-Cu and ε-bronze phases was supported with the Cu-Sn binary phase diagram (Fig. 4).Fig. 5 illustratess the SEM images of the microstructure of segments without boron carbide.As illustrated, the amount of pores in the segments sintered at 600 °C was more in comparison to that of 700 °C.The amount of pores decreased at a high sintering temperature, which lead to high speed solid-state diffusion [14].The pores formed at the grain boundary.The XRD patterns illustrate that there was no chemical reaction between bronze and BB 4 C.The level of porosity increased together with the increased rate of boron carbide because boron carbide had an adverse effect on sinterability [15].Table 1 illustrates the EDS analysis of regions identified in the SEM images illustrated in Fig. 6.The region 1, 2, and 3 illustrate B 4 B C, CuSn and Cu 3 Sn phases, respectively.Tab.I EDS analysis of regions identified in the SEM images illustrated in Fig. 6 Chemical

Density and hardness
Tab. II illustrates the effect of sintering temperatures and boron carbide on densities of segments.When boron carbide was introduced to the CuSn, it decreased the sintered density.This was due to the fact that the density of boron carbide was lower than that of bronze.The density of B 4 C was 2.52 g/cm 3 , while the density of bronze was 8.68 g/cm 3 .Relative density also decreased as the amount of added boron carbide increased.This was due to the fact that the increased rate of added boron carbide had an adverse effect on sinterability.Another reason was the fact that there was a great difference in the melting points of the bronze and the boron carbide, and boron carbide may have an inhibiting effect in the rearrangement of the grains during sintering [16].At higher sintering temperatures, a denser structure was formed due to higher diffusion rates [17].The difference between theoretical and sintered densities decreased with the increase in sintering temperature (Tab.II).
Tab. II The effect of sintering temperature and B 4 C content on the densities of the segment matrix.
Sintered density (g/cm As temperature increased, the two adjacent grains formed a good bond by diffusion in a solid-state bonding process, the relative and sintered densities of the segments increased [18].The highest compression for boron carbide added segments was obtained for the CuSn-2 % B 4 C segment and at 700 °C sintering temperature, with a relative density of approximately 93.02 %.   [19,20].The matrix hardness increased with the increase in sintering temperature.This was due to the same reason of the good bonding between the grains of the composites at a high sintering temperature [21].

Transverse rupture strength (TRS)
Fig. 8 illustrates the effect of sintering temperature and boron carbide content on the TRS.The TRS of the segments decreased together with the increase in the amount of boron carbide.This situation can be explained as the increase of boron carbide grains lead to the increase of the total area of the weakly bonded interface, which results in the decrease of the TRS [22].Furthermore, the level of porosity affects the TRS [23].With increasing of the sintering temperature, the TRS values of segments increased.This was due to good bonding between the bronze and B 4 C/diamond grains in case of high temperature [24].Fig. 9 illustrates the SEM images taken from the fracture surfaces of the segments containing 5wt.% BB 1 0 4 C.There was weak bonding between matrix and diamond.B 4 C grains adversely affected the bonding of diamond-matrix by getting bronze and diamond.This situation proved the presence of B 4 C grains in the diamond holes.The weak bonding was due to insufficient sintering conditions.The quality of bonding between diamond and matrix can be improved by changing sintering conditions.

Wear behavior
Fig. 10 illustrates the wear rate of diamond cutting segments versus the sintering parameters.The wear rate decreased significantly in the 0-5wt.%B 4 C contents, while it increased in the 5-10wt.%B 4 C contents.The addition of B 4 C to bronze powder increased the strength by a dispersion hardening mechanism, lead to a decrease in the wear rate [25].In this study, it is understood from wear rate values that the critical B 4 C content was 5%.When the boron carbide content was more than 5wt.%, the wear rate increased due to the insufficient sintering conditions, which lead to pull out boron carbide grains.These grains having abrasive property caused more wear of the matrix.SEM images of the worn surface of diamond cutting segments without B 4 C sintered at 600 and 700 °C are illustrated in Fig. 11.The micro ploughing grooves, the matrix tail, and the state of fractured diamond grains were formed in the segment.The SEM images clearly illustrate that a strong diamond-matrix bond occurred with increasing sintering temperature.SEM observations of worn surface of diamond cutting segments containing B 4 C are illustrated in Figs. 12 and 13.The SEM images illustrate the diamond and boron carbide grain distribution and the state of every diamond grains under these conditions.The distribution of diamond and B 4 C grains is relatively homogeneous.In the event that diamond is not homogeneously distributed, no uniform cutting is established on the surface of the segment, and no uniform wear is formed in the matrix.This is an issue that affects the operating performance and life of the diamond cutting tools.From observations of the worn surface of segments, the diamond grain can be classified as: whole diamond, polished (flattened) diamond, fractured diamond, and diamond pull out [26,27].There were fractured diamond grains on the wear surfaces of all segments.This fracture can be just a fracture in one corner of the diamond (Fig. 12a and 13a), or an intense fracture (Fig. 12b).These fractures in diamond grains can be caused by the micro fractures on the surface of the diamond grain and the crushed sections, as well as it losing strength under repetitive loads.Fig. 12b and 13b illustrate diamond grains that are not worn and have not lost any of their cutting corners.These grains display an effective cutting performance during cutting.Sometimes the diamond grains on the wear surface can be polished (flattened).The cutting edges of the diamond grain flatten during cutting, which causes them to loss cutting function, and make the cutting tool blunt (Fig. 13c).In the event that the bonding between the matrix and the diamond grain is not at the desired level, the diamond grains easily fall off as the matrix wears (Fig. 12c).
Under these circumstances the cutting tool becomes useless as a result of rapid wear.In general, the wear experienced by the matrix in diamond cutting tools is abrasive, erosion, and fatigue wear [28,29].The matrix is worn by the abrasive wear mechanism, and the diamond grains fulfill their cutting function.The matrix must wear at optimum level.As the swarfs of the natural stone being cut disperse from the cutting region, they form a crater-like structure around the diamond.This occurs as cavitation erosion wear during the cutting process (Fig. 13b).Fig. 13c illustrates the heap created by the swarf of the andesite stone around the diamond grain.These swarfs cause the cavitation erosion wear around the diamond as referred to above.Fig. 10 proves that the matrix is exposed to less wear when boron carbide is added to the bronze matrix.This can also be seen in the SEM images of wear surfaces illustrated in Figs.12-13.The groove depth in boron carbide segments is less in comparison to the groove depth of segments with no boron carbide.This is due to the fact that the matrix is harder.SEM images illustrate that more diamonds fall out of their holes with the increase in the boron carbide rate.In particular, the diamond holding capability of the 10% B 4 C added matrix is significantly low due to insufficient sintering conditions; the amount of diamond that falls out is higher in such matrix segments.The SEM images in Fig. 13 proved that the bonding between diamond and the matrix was stronger when the sintering temperature was increased to 700°C.SEM images also proved that diamond lose caused by weak diamond-matrix bonding was less for segments in this group.The hardness of the matrix, which has a more intense structure at a higher sintering temperature, increased and resulted in the matrix forming a strong bond with diamond grains.

Conclusions
Listed below are the major conclusions of this study: 1.The segments containing B 4 C were successfully produced using a hot pressing method together with a three-minute sintering at 600, 650, and 700 °C, under a pressure of 35 MPa. 2. Boron carbide grains formed at the grain boundary of bronze, and were surrounded by bronze.The increase in the amount of boron carbide added increased the amount of pores.The amount of pores decreased at high sintering temperature which leads to high speed solid-state diffusion.

Fig. 6
Fig. 6 illustrates the SEM images of the segments produced by adding 2%, 5%, and 10% B 4 C in weight to bronze powder.The B 4 C grains were relatively homogeneously distributed throughout the microstructure, and surrounded by bronze.In micrographs, light grey areas indicate bronze matrix, and the dark grey and cornered shapes indicate the reinforcement component B 4 C.As illustrated in Fig.6, as the sintering temperature and B 4 C addition increases, B 4 C grains spread towards the bronze grain boundaries, like a homogenous network.The XRD patterns illustrate that there was no chemical reaction between bronze and BB 4 C.The level of porosity increased together with the increased rate of boron carbide because boron carbide had an adverse effect on sinterability[15].Table1illustrates the EDS analysis of regions identified in the SEM images illustrated in Fig.6.The region 1, 2, and 3 illustrate B 4 B C, CuSn and Cu 3 Sn phases, respectively.

Fig. 7 .
Fig. 7.The effect of sintering temperature and B 4 C on the hardness of segment matrix.

Fig. 7
Fig. 7 illustrates hardness as a function of the sintering temperature and the B 4 C content in the segments.Hardness values were determined by taking the average of six

Fig. 8 .
Fig. 8.The effect of sintering temperature and B 4 C on the TRS of segments.

Fig. 9 .
Fig. 9.The SEM images of the fracture surfaces of the segments having bronze-5 wt.% B 4 C matrix: (a) sintered at 600 °C and (b) sintered at 700 °C.

2 Fig. 10 .
Fig. 10.Effect of the sintering parameters on the wear rate of the segments.