Papers by Author: Rômulo Crespo Tardim

Abstract: This paper presents the results on the strength evaluation of sintered diamond-based composites with Co binder. The method of intensive electrosintering with a short heating pulse was applied by a high pressure of 0.5 GPa and a time of 2-40 s. Composites with different compositions and for different purposes, such as rock drilling and dressing tools, were investigated. The reactive cell was design with the purpose of preserving the diamond properties from the detrimental sintering temperature exposure. The results of this electrosintering technology permitted to obtain improved composites to be used in tools with higher indexes of performance and productivity.

Abstract: One way to develop a new composite material or to improve its performance is to reinforce the matrix with a stronger dispersive phase. In principal, nanodiamonds can be used as reinforcement of composites owing to high surface energy, which allows an effective structure interaction with most utilized matrices. In this work, polycrystalline metallic–based composites and polymeric-based composites were investigated for the effect caused by addition of nanodiamonds. These composites were obtained at high compression pressures and combine with temperatures for certain short processing times. It was found that even small additions of nanodiamonds increase the wear resistance of the investigated composites.

Abstract: The present work comprises a study about the possibility of obtaining polycrystalline diamond cutters through a novel method of sintering both layers at the same time. This possibility was tested through the sintering of a diamond layer over a hard metal (WC+15wt%Co) support under conditions of 5.0 and 6.5 GPa of pressure and 1400 to 1600º C of temperature. The sintering conditions were imposed in two ways: directly, or with pre-sintering. The samples were tested by measuring microhardness, wear resistance, densification, and SEM. The results of the tests have shown the possibility of obtaining good quality inserts by sintering both layers of compacted powder.

Abstract: In the rock-drilling industry, double layer polycrystalline diamond-hardmetal (Dia-HM) inserts for cutting tools are subjected to elevated levels of wear due to the high power involved in the process. A research performed by focusing on the wear mechanism has shown that the complexity of the actual industrial operation can be reproduced in laboratories. The present work developed a methodology to calculate the theoretical values of wear of drilling tools. The main parameter selected for this methodology was the amount of disaggregated rock. The information acquired through this methodology gives one a complete perspective about the application of Dia-HM inserts to be used in drilling tools.

Abstract: Polycrystalline SiC-diamond composites have been fabricated by high pressure and high temperature, HPHT, sintering conditions using a Si infiltration method. However, infiltration of liquid Si around the diamond particles results not only in SiC but also in free Si, which causes deterioration of the composite properties. In this work, a novel sintering procedure was developed to avoid the formation of free Si in the composite structure. A disk composed of a mixture of graphite and Si was first press-molded at room temperature. The disk was then placed above the diamond powder inside a high pressure chamber used for the HPHT sintering process. This arrangement permitted to preferentially form liquid SiC, which infiltrates in between the diamond particles. Using this procedure, free Si formation is inhibited and the SiC-diamond composite forms a rigid structure with improved properties.

Abstract: Inserts of drill bits used in perforation of wells are employed to cut many different kinds of stone. The material that shows the best performance on this application is the WC+Co+Diamond composite, obtained via powder metallurgy. However, heterogeneous microstructural aspects in these composites may impair their efficiency. On this work, WC+6%Co-based composites were obtained via high pressure sintering at 5.0 GPa, with diamonds, WC and Co powders. The particle size of the diamond was 400/315 μm, and for the WC and Co, 100/63 μm. Part of the samples also received 2wt%CrB2 as a doping agent. Wear tests were carried out in an abrasimeter with a maximum axial load of 50 kg. Linear and volumetric wear indices achieved values of 821∙10-6 g/m and 10.7 g/m3, that are superior to inserts produced via conventional powder metallurgy.

Abstract: The technologies of superhard materials production, including those for industrial synthesis of diamond and cubic boron nitride at high pressure and high temperature conditions, depend on the operator experience. Moreover, these manually operated processes usually vary in terms of yield and quality of the final product. Owing to the design and construction methods of conventional high pressure devices (HPD), it has not yet been possible to adapt computerized systems for both research and industrial production. In this work, the synthesis of superhard materials was treated as a guided problem under uncertain pressure, temperature, time, and electrical power conditions using the theory of automatic commanded systems. This concept was applied to develop an automatic control for the hydraulic presses that generate the necessary force for the high pressure, based on variable sensors associated with computer programs that command the conditions inside the HPD.

Abstract: Powder metallurgy-based technologies of saw blades production for stone cutting normally uses cobalt as binder agent. Cobalt was chosen for that purpose because it provides improved properties, high wettability and good adhesion between dispersed diamond particles. In addition to its excellent physical properties, cobalt also shows a positive behavior associated with the control of diamond graphitization at temperatures up to 1000°C. In this work, an experimental planning method supported by a mathematical algorithm was used to study the influence of doping agents incorporated into cobalt-based binders. The consequences on the wear resistance of saw blades, as well as on other physical and mechanical properties of the “diamond-cobalt-doping agent” composites were investigated. Cr3C2, Si, Ni and Fe were used as doping agents. Experimental tests were carried out using granite as a base material for wear and cutting operations.

Abstract: Nanometric powders of WC with 10 weight% Co were mixed in high-energy mill. Compaction was performed at 200MPa and processed by the technique of high pressure and high temperature (HPHT). Sintering conditions were P = 5GPa, T = 1300-1400-1500° C, t = 2-4 min. For comparative purposes, samples were conventionally sintered at T = 1400° C, t = 45 min, vacuum of 10-2 mbar.

Abstract: Polycrystalline SiC-diamond composites have been fabricated by high pressure and high temperature, HPHT, sintering using a Si infiltration method. However, infiltration of liquid silicon around the diamond particles results not only in SiC but also in free silicon, which causes deterioration of the composite properties. In this work, a novel sintering procedure was developed to avoid the formation of free silicon in the composite structure. A disc composed of a mixture of graphite and silicon was first press-molded at room temperature. The disc was then placed above the diamond powder inside a high pressure chamber used for the HPHT sintering process. This arrangement permitted to preferentially form liquid Si in a C solution, which infiltrates in between the diamond particles. Using this procedure, free silicon formation is inhibited and the SiC-diamond composite forms a rigid structure with improved properties.