Piotr Siwak

Evaluation of Cutting Edges Made of Nanocrystalline Cemented Carbides Sintered by the Pulse Plasma Method

In our investigations, nanocrystalline WC-5 wt % Co was consolidated by the pulse plasma sintering method at various temperatures between 1320 and 1560 K under a pressure of 60 MPa for 300 s. The cemented carbides sintered at 1520 K have a relative density of 100 %, hardness of 2100 HV30, and tungsten carbides WC crystallite size of about 150-300 nm. An increase of the sintering temperature to 1560 K results in the increase of the WC crystallite size to about 300-500 nm and the hardness being decreased to 1980 HV30. The tool life of the cutting edges made of nanocrystalline cemented carbides sintered by the pulse plasma method is increased about two times in comparison with cutting edges made of standard and fine-grained cemented carbides during the turning of EN1.45.40 1H18N9T austenitic steel. On the basis of the literature data 1-4 and our own investigations 5-8, a decrease reduction of the tungsten carbides WC grain size in WC-Co cemented carbides increases the fracture toughness, bending strength, and hardness. WC-Co cemented carbides are most often produced by sintering with the partici- pation of a liquid cobalt phase. The presence of a liquid cobalt phase during the WC-Co sintering causes the growth of the WC grains. The growth of the WC grains is due to the high rate of diffusion of WC through the liquid cobalt phase. In the newest sintering methods; for example, spark plasma sintering SPS, field assisted sintering FAST, and plasma assisted sintering PAS9-11, the sintering process is very short and carried out at a lower temperature than in the conventional methods. A characteristic feature of the SPS, FAST, and PAS methods is a current pulse for heating the powders during sintering. Spark discharges during a current pulse are ignited in the pores. The discharges formed in the pores remove absorbed gases and oxides from the surface of powder particles, thereby facilitating the formation of active contact between them. The present investigation was concerned with sintering nanocrystalline WC-5 wt % Co powders using a new pulse plasma sintering PPS method 12. As in the SPS, PAS, and FAST methods, in the PPS method the spark discharges during a current pulse are ignited in the pores. The phenomena occurring in the PPS process are shown in Fig. 1. In this article, the results of a comparative investigation of the durability of cutting edges made of nanocrystalline WC-5 wt % Co cemented carbides sintered by the pulse plasma method and cutting edges made of standard and fine-grained cemented carbides 1.5 m of the same chemical composition during the turning of EN1.45.40 1H18N9T austenitic steel are presented. Austenitic steel, widely applied in the food and chemical industry, belongs to hard machinable materials 13. The hard machinability of auste- nitic steel comes from a surface hardening phenomenon during machining. Therefore, the machining is carried out with very low cutting parameters. For this reason, the investigations of nanocrystalline ce- mented carbides were taken into consideration. The nanocrystalline cemented carbides are significantly harder and more wear resistant then the standard or fine-grained cemented carbides 4-6.

Influence of Structure on Brittleness of Boron Nitride Coatings Deposited on Cemented Fine-Grained Carbides

The article presents the brittleness study of boron nitride coatings deposited on cutting edges made of fine-grained cemented carbides by the pulse-plasma method (PPD). Influences of the structure (density, pores, microcracks) of coating material on the brittleness and on selected technological parameters of boron nitride formation by PPD method particularly taking into account discharge voltage on brittleness are shown. Differences between values of total average crack length ΣL, critical loads (Pk300, Pk500), and coefficients (a1(300) and a1(500)) characterized susceptibility to cracking of investigated coatings manufactured using different values of production process were defined. Results of investigations have confirmed the usefulness of Palmqvist’s method for measurement of coating susceptibility to brittle cracking.

STUDY OF THE EFFECT OF THE ELECTROLESS Ni-P COATING THICKNESS APPLIED ON AW-7075 ALUMINUM ALLOY ON ITS MECHANICAL PROPERTIES

The paper presents the results of examination of microhardness of chemical coatings Ni-P, applied on samples of aluminium alloy for plastic processing AW-7075. The examinations have been performed on samples with three different thicknesses of the Ni-P coatings; next the influence of their thickness on the depth of material penetration by the indenter under load, as well as its influence on creep, increase of Martens hardness and on the Young’s modulus of the system of the coating with the substrate material. Hardness measurements have been performed with the use of PICODENTOR HM500 nanohardness tester (Vickers). The purpose of the examination of Ni-P coating deformation without dispersion phases is analysis of their mechanical properties in respect of the possibility of technical applications on the aluminium alloy AW-7075 (among others, by the selection of optimum thicknesses on hard aluminium alloy), as well as the determination of the possibility of applying dispersion phases in order to improve their properties.

Formation and Properties of the Ta-Y2O3, Ta-ZrO2, and Ta-TaC Nanocomposites

The nanocrystalline tantalum-ceramic composites were made using mechanical alloying followed by pulse plasma sintering (PPS). The tantalum acts as a matrix, to which the ceramic reinforced phase in the concentration of 5, 10, 20, and 40 wt.% was introduced. Oxides (Y2O3 and ZrO2) and carbides (TaC) were used as the ceramic phase. The mechanical alloying results in the formation of nanocrystalline grains. The subsequent hot pressing in the mode of PPS results in the consolidation of powders and formation of bulk nanocomposites. All the bulk composites have the average grain size from 40 nm to 100 nm, whereas, for comparison, the bulk nanocrystalline pure tantalum has the average grain size of approximately 170 nm. The ceramic phase refines the grain size in the Ta nanocomposites. The mechanical properties were studied using the nanoindentation tests. The nanocomposites exhibit uniform load-displacement curves indicating good integrity and homogeneity of the samples. Out of the investigated components, the Ta-10 wt.% TaC one has the highest hardness and a very high Young’s modulus (1398 HV and 336 GPa, resp.). For the Ta-oxide composites, Ta-20 wt.% Y2O3 has the highest mechanical properties (1165 HV hardness and 231 GPa Young’s modulus).

Application of Cutting Edges with High Durability Made of Nanocrystalline Cemented Carbides

This chapter presents the results of the wear and durability investigations of cutting insert edges made of Nano_WC-5Co and Nano_WC-5Co + Cr3C2 nanocrystalline sintered carbides. Cutting inserts were sintered using nanocrystalline powder by the pulse-plasma method. Durability of the cutting edges was determined during turning of EN 1.45.41 (1H18N9T) austenitic steel. The Nano_WC-5Co + 0.9% Cr3C2 nanocrystalline cemented carbides have much higher hardness and smaller average grain size than the Nano_WC-5Co nanocrystalline carbides without growth inhibitor. For these reasons, cutting inserts made of the nanocrystalline cemented carbides with 0.9% Cr3C2 by weight exhibited significantly greater resistance to wear and greater durability during machining of austenitic steel.

Hot pressing of nanocrystalline tantalum using high frequency induction heating and pulse plasma sintering

The paper presents the results of nanocrystalline powder tantalum consolidation using hot pressing. The authors used two different heating techniques during hot pressing: high-frequency induction heating (HFIH) and pulse plasma sintering (PPS). A comparison of the structure, microstructure, mechanical properties and corrosion resistance of the bulk nanocrystalline tantalum obtained in both techniques was performed. The nanocrystalline powder was made to start from the microcrystalline one using the high-energy ball milling process. The nanocrystalline powder was hot-pressed at 1000 °C, whereas, for comparison, the microcrystalline powder was hot pressed up to 1500 °C for proper consolidation. The authors found that during hot pressing, the powder partially reacts with the graphite die covered by boron nitride, which facilitated punches and powder displacement in the die during densification. Tantalum carbide and boride in the nanocrystalline material was found, which can improve the mechanical properties. The hardness of the HFIH and PPS nanocrystalline tantalum was as high as 625 and 615 HV, respectively. The microstructure was more uniform in the PPS nanomaterial. The corrosion resistance in both cases deteriorated, in comparison to the microcrystalline material, while the PPS material corrosion resistance was slightly better than that of the HFIH one.