Hardness testing was done using Vickers hardness machine. The result shows that the ... shows a ligament shape of particles (Fig. 1a). The Cr had an irregular ...
Effect of alloying elements (Ni and Cr) on density and hardness of Co alloy fabricated by powder metallurgy Raudhah Othman1, Shamsul Baharin Jamaludin 2, Mohd Nazree Derman 3 1
Department of Design and Materials Engineering, Faculty of Materials and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja, Johor. 2 School of Materials Engineering, Universiti Malaysia Perlis, Komplek s Pusat Pengajian Jejawi 2, 02600 Jejawi, Arau, Perlis. 3 Institute of Nano-Electronic, Universiti Malaysia Perlis, Komplek s Pusat Pengajian Jejawi 2, 02600 Jejawi, Arau, Perlis.
Abstract Influences of Ni and Cr addition on the properties of powder metallurgy Co base alloys were investigated. The alloys were produced by powder metallurgy technique and sintered under argon atmosphere. Scanning electron microscope (SEM) was used for morphology observation of the raw materials. The size of the Co, Cr and Ni particles has been characterized using particle analyzer. Density measurement was carried out by Archimedes technique while porosity was calculated based on density data. Hardness testing was done using Vickers hardness machine. The result shows that the bulk density of the alloys decreased with increasing of Cr. Hardness of CoCrNi alloy decreas es with increasing of the Ni content. Keywords: Co-Cr-Ni, density, porosity, microstructure 1. Introduction Materials used in medicine should fulfill several strict conditions primarily imposed by biological restrictions: absence of harmful interactions with body cells, good compatibility with organ and, finally, suitable mechanical properties [1]. The success of a biomaterial in the body depends on factors such as the material properties, design, and biocompatibility of the material used, as well as factors not under control of the engineer, including the technique used by the surgeon, the health and condition of the patient, and the activities of the implant. Biocompatibility can be explained as an acceptance of an artificial implant by surrounding tissues and by the body as whole. The implant should be compatible with tissues in terms of mechanical, chemical, surface, and pharmacological properties [2, 3, 4]. There are three categories of materials presently used in prosthetic devices which are metals, polymers, and ceramics. Biometals used in orthopedic implants include surgical grade stainless steel, cobalt-chromium alloys, titanium, and titanium alloys. Stainless steel is not suitable for a permanent implant because of its poor fatigue strength and liability to undergo plastic deformation, yet is still commonly used for nonpermanent implant such as internal fixation devices for fractures. Before the use of titanium, cobalt (Co)-based alloys had largely replaced stainless steel as materials for permanent implants. These alloys are generally more corrosion resistant owing to the formation of a durable chromium-oxide surface layer or passivation layer [5]. Chromium (Cr) as well as molybdenum (Mo) has an ability to improve the corrosion resistance of alloys. Besides they act as tungsten stabilize the hcp-structure of Co-matrix which is important to improve the mechanical properties, reduce the abrasive wear and lower the stacking faults energy. The formation of stacking faults diminishes the ductility of Co alloys. However, Co-Cr alloy experienced a low ductility which by alloy additions, their properties could be improved. By adding nickel (Ni), it can reduce the tendency to form stacking faults which improves ductility and stabilizes fcc-structure of Co-matrix [5]. A series of powder metallurgy Co-Cr alloy with Ni addition has been investigated in order to obtain the alloy with optimal properties including density, porosity and hardness.
2. Materials and methods Co, Cr and Ni powders were mixed together in planetary ball mill (Recth, Pm100) at 450rpm for 30 minutes. A series of eleventh different composition of Co-Cr-Ni mixture were prepared and was showed in Table 1. Prior to mixing, raw materials particles were characterized by particle analyzer to determine its particle size. The Scanning Electron Microscope (SEM) (JSM-6420, JEOL) was used for the morphology observation. Alloys of Co, Cr and Ni were compacted using a uniaxial press at pressure of 240 MPa at room temperature. The samples were sintered under argon atmosphere using tube furnace. The heating rate was set to 3°C / min until it reaches 400°C and soaked for 1 hour. The heating rate for sintering process was then set up to 5°C / min until it reaches 1000°C and soaked for 2 hours. The density measurement of CoCrNi alloy samples were determined using Archimedes method. Porosity was calculated based on density data. The hardness property was studied by using Vickers method (Mitutoyo DX256). The applied force was set at 0.3N for 10 seconds. Samples for hardness test were first ground with 240-600 grit SiC paper. Table 1 The composition (in weight percent) of the experimental Co-Cr-Ni alloys Sample no. Composition (wt. %) Co50Cr50 1 Co55Cr5Ni40 2 Co57Cr20Ni23 3 Co58Cr37Ni5 4 Co61Cr9Ni30 5 Co62Cr28Ni10 6 Co68Cr15Ni17 7 Co73Cr4Ni23 8 Co75Cr20Ni5 9 Co81Cr9Ni10 10 Co91Cr4Ni5 11 3. Results and discussion In order to examine the effect of nickel and chromium addition on the properties of Co alloys, 11 samples with different Co, Cr and Ni composition were fabricated. The raw material characterization of Co-Cr-Ni powder was done including microstructure observation and particle size characterization. From the Scanning Electron Microscope (SEM) observation as shown in Figure 1, the Co particles shows a ligament shape of particles (Fig. 1a). The Cr had an irregular shape while nickel shows a spherical shape of the particles (Fig. 1b & c). For the particle size analysis, Co powder had a bimodal distribution with mean size of particle of 994µm (Fig. 1d), while Cr and Ni powders shows a monomodal distribution with mean size of particles of 151 µm and 12 µm respectively as shown in Figure 1 (e&f).
a
d
b
c
e
f
Fig. 1: the SEM results of raw (a) cobalt (b) chromium (c) nickel alloy; the particle size graph for (d) cobalt (e) chromium (f) nickel powders The bulk density for all samples in Figure 2 shows an increased trend which sample 1 with the highest amount of Chromium addition had the lowest density while sample 11 with the lowest Chromium content had the highest density. Comparison was made with the rule of mixture calculation results the same increasing trend. The data collected for bulk density testing shows that the density of CoCrNi alloy was decreased with the increasing of Cr content.
Fig. 2: The bulk density for 11 samples CoCrNi.
Fig. 3: The hardness of CoCrNi samples. Figure 3 shows the hardness results for all samples. Hardness is a measure of the resistance of a metal to permanent (plastic) deformation. The hardness of a metal is measured by forcing an indenter into its surface. The hardness of a metal depends on the ease with which it plastically deforms. Thus a relationship between hardness and strength for a particular metal can be determined empirically [6]. The size of the microstructure of pure metals and single phase alloys, cold work and recrystallization of cold-worked metals all affect the determination and value of hardness. The hardness of very pure metals is shown to be directly related to their melting point [7]. As can be observed from the bar chart plotted, sample 2 with the highest weight percent of nickel content has the lowest hardness. Conversely sample 10 with the lowest percentage of nickel has the highest hardness. The importance of doing the hardness test is to identify the optimum composition having the best properties as it is related to the
value of hardness. Form the testing conducted including density, porosity and hardness test, the correlation among those testings showed that as the density of the sample increased, the porosity of samples decreased, relating to the increment of the hardness of samples. 4. Conclusion Studies on the effect of alloying elements nickel and chromium on the density, porosity and hardness of powder metallurgy Co-Cr-Ni alloy was performed in order to determine the region of their optimal properties. The main results were as follows: 1. The bulk density of CoCrNi alloy decreased with increasing amount of chromium. 2. The hardness test shows that the hardness of CoCrNi samples decreases with increasing of nickel content. 5. Acknowledgements This study was sponsored by Sciencefund grant no. 9005-00008. References [1] J. Jagielski, A. Piatkowska, P. Aubert, L. Thome, A. Turos, and A. Abdul Kader, Ion implantation for surface modification of biomaterials, Surface and Coatings technology, 200, 2005, pp 22-23. [2] Joon B. Park and Roderick S. Lakes, Biomaterials an introduction, 2nd Edition, Plenum Press, New York, 1992. [3] Joyce Y. Wong, Joseph D. Bronzino, Biomaterials, CRC Press, London, 2007. [4] Seeram Ramakrishna, Zheng-Ming Huang, Ganesh V Kumar, Andrew W Batchelor, Joerg Mayer, An Introduction to Biocomposites, Vol.1, Imperial College Press, London, 2004. [5] Tanja Matkovic, Prosper Matkovic, Jadranka Malina, “Effects of Ni and Mo on the microstructure and some other properties of Co-Cr dental alloys”, Journal of Alloys and Compounds 366, 2004, pp 293297. [6] William F. Smith, Foundations of Materials Science and engineering, Third edition, Elizabeth A. Jones, New York, 2004. [7] H.T. Angus, “The significance of hardness”, Wear, 54, 1979, pp 33-78.
