Characterization of the elastic modulus of ceramic – metal composites with physical and mechanical properties by ultrasonic technique

: The scope of this study, that is, the e ﬀ ect of the elastic modulus obtained by ultrasonic method on the physical and mechanical properties of tungsten carbide ( WC )- based ceramic – metal composites, which have Ni and Co metallic binder composition produced by powder metallurgy and represented by high strength and hard - ness criteria, was investigated. In order to obtain compos - ite samples in the study, it was sintered in a microwave furnace at di ﬀ erent temperatures to combine the powder particles prepared at the rate of 60% Ni, 20% Co, and 20% WC by weight. Then, the velocities and longitudinal attenuation values of longitudinal and shear ultrasonic waves along the composite sample were measured using the ultrasonic pulse - echo method. The elastic modulus of the composites was determined using ultrasonic veloc - ities and sample density. Hardness testing, scanning electron microscopy ( SEM ) , and X - ray di ﬀ raction ( XRD ) analyses were also performed. The results show that the elastic modulus increases with the increase in sintering temperature and ultrasonic wave speeds, but decreases with the longitudinal attenuation value, considering the SEM images and XRD analysis. There is also a linear rela - tionship between elastic modulus and sti ﬀ ness.


Introduction
The field of materials science and characterization is to create new materials under different physical conditions, develop, and improve previously known materials [1].Today, the quality of WC-reinforced ceramic-metal composites produced in powder metallurgy is expressed by their high strength and hardness values.In addition, composite materials with WC are known to be used in places requiring high strength [2][3][4][5][6].The subject of material characterization is mainly the study of elastic behavior, material microstructure, and morphological properties, and relevant mechanical properties and includes evaluation.Destructive and non-destructive testing techniques are available for full material characterization without damaging the structure [7].In recent years, it is possible to characterize the microstructural, physical, and mechanical properties of materials using ultrasonic testing methods.Research studies and development of ultrasonic testing procedures and equipment are carried out, trying to achieve better results.Each ultrasonic test parameter is significantly affected by changes in the microstructural or mechanical properties of the materials.Quantities, ultrasonic velocity, and attenuation are important parameters required for the ultrasonic nondestructive material characterization technique [8][9][10].Ultrasonic velocity gives information about the mechanical, anisotropic, and elastic properties of the medium through which it passes, while at the same time it is related to the elastic constants and density of a material [11,12].In addition, as a result of the studies, it has been observed that the ultrasonic method is a very sensitive and reliable technique from the data obtained for sintered composite samples and materials [13][14][15][16][17][18].
According to the literature, materials obtained by microwave sintering can exhibit comparable and highlevel properties under the conditions of appropriate chemical composition and production technology, in specific application conditions superior to the observed properties of many hard metals, alloys, cermets, and composites.Microwave sintering has been widely used in the field of powder metal technology [19,20].Compared to other conventional sintering methods, microwave sintering is an economical heating method that saves time and energy and also limits grain growth with its fast-heating rate.It provides better production performance with a good microstructure and therefore improved mechanical properties [21][22][23][24].All these advantages make it one of the scientific and economical and industrially practical methods for the preparation of hard metals such as nickel (Ni), cobalt (Co), and tungsten carbide (WC) to be formed by microwave heating.Besides this kind of work, there are many kinds of works done on material science to investigate different properties .
In this study, ceramic-metal composites were sintered at different sintering temperatures.The aim is to measure the elastic modulus of composites prepared at different temperatures ultrasonically and to examine the relationship between ultrasonic properties and physical and mechanical properties without damaging the structure in order to explain the changes in the structure.In the study, the velocity and attenuation values of the samples and the modulus of elasticity were measured using ultrasonic method, microstructural development was examined with a scanning electron microscope (SEM) and X-ray diffraction (XRD), and hardness measurement was made for hardness values.

Experimental procedures 2.1 preparation of sample
In this study, Ni, Co, and tungsten carbide (WC) were used in the form of powders.The powders used in the study were 99.8% purity and particle size Ni from lower than −325 mesh, Co powders with 99.9% purity and a particle size of lower than 150 μm, and WC powder with 99.9% purity and a particle size of lower than 50 μm.All powders were obtained from Sigma Aldrich.The powders prepared by weight (60% Ni-20% Co-20% WC) were mixed homogeneously in a mixer for 24 h after weighing.All the powders were placed in a mold of 15 mm diameter and 5 mm height and pressed using a hydraulic press at a pressure of 305.9 kg/cm 2 , and then, the cold-pressed samples underwent sintering at 600-1,000℃ for 2 h in a microwave sintering.The sintering process was carried out in a Phoenix Standard Unit (CEM Innovations in Microwave Technology) brand microwave oven with a heating rate of 10°C per minute.

Mechanical characterization
Microhardness measurements of Ni-Co-WC composite specimens were made using a METTEST-HT (Vickers) brand microhardness tester by applying Vickers hardness at 0.5 kg load.In addition, in order for the results of the hardness measurements to be accurate, measurements were taken from 10 different regions of each composite sample, and the hardness value was given with an average value.
Shimadzu brand XRD-6,000 X-Ray diffraction analyzer was operated with Cu K alpha radiation at a scanning rate of 2°per minute.
Equipped with X-ray (energy-dispersive X-ray -EDX spectroscopy) detector, LEO-1,430 VP SEM was applied to observe the microstructure of composite material.
The densities of the prepared composite samples of the (60% Ni-20% Co-20% WC) composition were calculated according to the Archimedes principle, taking into account the volumetric changes after sintering at different temperatures.

Ultrasonic measurements
In ultrasonic measurement methods, as in every experimental system, there is a transducer that produces the ultrasound and another transducer that detects the produced ultrasound from the other end of the environment.In our experimental study, we used pulse-echo method, one of the ultrasonic measurement methods.The ultrasonic pulse-echo technique is generally used to take precise measurements of ultrasonic velocity and absorption in the megahertz and gigahertz frequency regions, to evaluate its elastic modulus, determine microstructure characterization, and evaluate its mechanical properties [52].Velocity and attenuation measurements of the samples were made with Sonatest Sitescan 150 model ultrasonic wave flaw detector.A 2 MHz (Sonatest SLH2-10, T/R) transmitter/receiver longitudinal probe was used to measure ultrasonic longitudinal velocity, and 4 MHz (GE Inspection Technologies MB 4Y 66100541) transmitter/receiver shear probe was used to measure ultrasonic shear wave velocity.Sonatest sonagel-W liquid gel was used as a coupling fluid between the probes and the sample.Then, the image of the ultrasonic wave sent to the sample with the transmitter/receiver transducer was obtained with the front wall reflection, back wall reflection peaks and echoes peaks on the A-Scan screen.By changing the height of the transducers from the sample, measurements were tried to be taken until the sharpest peak in the spectrum was obtained.The propagation rate of ultrasonic waves through the thickness of the samples was evaluated as follows: where d is the sample thickness (mm), t is the propagation time of ultrasonic wave (ns), and V is the propagation speed of the ultrasonic wave (m/s) [53,54].Considering that the samples obtained in this analysis are isotropic, Young's modulus can be calculated by using the standard velocity-elasticity formulas and the density of the sample.In this case, Young's modulus is calculated from the measured ultrasonic longitudinal (V L ) and transverse (V T ) wave velocities and density (ρ) [55] as follows: Also, while the ultrasound wave propagates in a solid environment, the term absorption is used for the average energy loss that occurs due to its direct interaction with the particles in the environment.This reduction event occurs due to the absorption and scattering properties of the solid and is called "attenuation."Using a flaw detector, the attenuation coefficient is generally obtained by the ratio of the amplitudes of the first back-wall echo to that of the second back-wall echo and echo peaks on the A-Scan screen [56].This formula is used for calculating the attenuation coefficient as follows: where A 1 and A 2 are the amplitude of two consecutive back wall echoes and d is the thickness of the samples in (mm).Also, in Figure 1, the general schematic representation of the experimental setup used for the ultrasonic measurements of the samples with the applied pulse-echo test is given.

Results and discussion
The 60% Ni-20% Co-20% WC composite samples formed with the powders prepared in certain proportions were sintered in a microwave furnace at temperatures ranging from 600 to 1,000°C.The main focus of this study is to investigate the effect of the elastic modulus obtained using ultrasonic method on the physical and mechanical properties of these ready-made composite samples.The ultrasonic velocity, ultrasonic longitudinal attenuation and elastic modulus values depending on the sintering temperature are shown in Table 1.In the study, graphs showing the change of physical-mechanical properties such as hardness, sintering temperature (°C), ultrasonic velocity values, and longitudinal attenuation of the samples given in Table 1 against elastic modulus values were drawn (Figure 2).Ultrasonic velocity in powder compaction processes in powder metallurgy is directly dependent on the density and elastic modulus.In order for ultrasonic velocity to realize its full potential as a parameter for the characterization of sintered materials, a better understanding of the relationship between ultrasonic velocity and powder compact state is required [57].When the velocities of both ultrasonic longitudinal and shear waves at 600°C sintering temperature and 1,000°C sintering temperature are examined, it is seen that there is an increase of approximately two times.It has been observed that a Characterization of the elastic modulus of ceramic-metal composites  595 more stable and dense structure can be obtained as the sintering temperature increases.In our study, it was found to be consistent with similar results in the literature.In addition, the fact that the velocity is high in low-density samples and low in high-density samples is a result of the measurements being made parallel to the pressing direction.As the temperature increases, the grain boundary movement speed increases, and because   Characterization of the elastic modulus of ceramic-metal composites  597 the pores move more slowly at the grain boundary, densification increases.For a sintering temperature of 1,000°C, the wave velocities are independent of the heating rate and reach their maximum values.Longitudinal and shear wave velocity and elastic modulus values vary in proportion to the total hardness values (Table 1).It is known that the existing pores in the structure block the path of incoming ultrasonic signals and thus reduce the velocity of ultrasonic waves [58].Therefore, it can be easily said that the total porosity values are low for a structure with high velocity and elastic modulus values.At the same time, the ultrasonic velocity measurement method appears to be an easy, convenient and suitable non-destructive technique for estimating porosity in materials [59].The relationship of Ni-Co-WC ceramic-metal composite samples with the elastic modulus is clearly shown in the graphs drawn.
While the elastic modulus decreases with the increase in the longitudinal attenuation polynomial (Figure 2c), it increases with the sintering temperature (Figure 2a) and the longitudinal and shear velocity values (Figure 2b).The results also show that the elastic modulus values increase with the increase in the hardness values (Figure 2d).Hardness, which is actually a type of strength, is largely controlled by its microstructure and the presence of imperfections.It is used to detect invisible discontinuities in materials or open discontinuities on the material surface using the pulse-echo method, which is one of the nondestructive testing methods.In this way, the microstructural, physical, and mechanical properties of 60% Ni-20% Co-20% WC composite samples were evaluated.

Metallographic and XRD analysis
SEM and XRD analysis results of metal matrix composite sample obtained from powders (60% Ni-20% Co-20% WC) sintered at 600°C and 1,000°C are shown in Figures 3 and 4.
Sintering is not better understood at (60% Ni-20% Co-20% WC) composition at 600°C temperature.The SEM picture (Figure 3b and c) shows some porous unclear phases.
In Figure 3d, the interaction between the particles has started and the structure is getting denser, the existing grain boundaries are gradually disappearing.Figure 3e is an SEM photograph taken at 1,000°C of the produced sample.It shows the start of the neck formation between particles.Grain boundaries are unclear.It is the moment when the porosity is less.Density is also a property that depends on the amount of porosity.In this case, there is a structure that becomes more stable with necking and the hardness value is the highest.After SEM analysis of the samples produced at 600, 700, 800, 900, and 1,000°C, X-ray analysis was made of the samples sintered at 600 and 1,000°C (Figure 4).The strong tungsten carbide peak according to the phases and elements, which emerged according to the XRD analysis taken at both temperature values, shows that the structure is composed of tungsten carbide and that WC, WO 2 , WO 3 , CoWO 4 , and W 2 C formations occur and the existence of the WC-Co system.Among these peaks, the W 2 C phase was not observed in the Ni-Co-WC composite sample sintered at 1,000°C, which is shown in Figure 4b.The obstacles to using the WC phase alone in the WC-Co system are its very brittleness and extreme difficulty in sintering.WC particles embedded in the cobalt phase give the structure its hardness and wear resistance, while the cobalt phase gives toughness and holds the WC particles together.In addition, in structures such as WC-Co, the stiffness is affected by the amount of the W 2 C phase [60].The disappearance of this phase at 1,000°C supports the high elastic modulus and hardness value.

Conclusion
Ni-Co and WC mixture powders were produced samples and were investigated.Metal powders are reinforced with ceramic powder.In the study, 60% Ni-20% Co-20% WC composite samples produced at different temperatures were characterized, and the ultrasonic pulse-echo technique was effectively used for the characterization of their properties.
Unlike conventional sintering, microwave sintering was used in the preparation of samples in the study.The reason is that since the heating of the sample in microwave heating is provided in the interior, it causes volumetric and homogeneous heating.Thus, the structural properties of the obtained sample are improved in many ways.At this point, the relationship between the elastic modulus of Ni-Co-WC ceramic-metal composites obtained by the ultrasonic method and their physical and mechanical properties was investigated.The results of the study show that an elastic modulus measurement is a reliable tool for the characterization of hardness, sintering temperature, ultrasonic longitudinal and shear velocity, and longitudinal attenuation in Ni-Co-WC composites.The highest density, hardness, and elastic modulus values of 60% Ni-20% Co-20WC composite produced by the powder metallurgy method were found as 6.663 gr/cm 3 , 301.69 HV, and 65.33 GPa, respectively, in the sample sintered at 1,000°C.Besides the desired microstructure, the highest hardness value obtained also brings good strength and stiffness.While the elastic modulus value increased approximately four times at 1,000°C, the longitudinal attenuation value decreased from 0.567 dB to 0.217 dB.The reason for this is that energy is lost as it interacts with more particles in the environment.Since the ultrasonic wave sent to the structure is absorbed by the particle, a reduction occurs depending on the interaction.

Figure 2 :
Figure 2: The relationship between E values and the physical-mechanical properties of composites.

Table 1 :
Physical and mechanical properties and sintering temperature of Ni-Co-WC ceramic-metal composites