Development of a New PM Tool Steel for Optimization of Cold Working of Advanced High-Strength Steels

In the present study, Uddeholm Vancron SuperClean cold work tool steel was investigated concerning wear resistance and fatigue strength, using laboratory and semi-industrial tests. The Uddeholm Vancron SuperClean was designed with the help of ThermoCalc calculations to contain a high amount of a carbonitride phase, which was suggested to improve tribological performance of this tool steel. In order to investigate the tested steel, galling tests with a slider-on flat-surface tribotester and semi-industrial punching tests were performed on an advanced high-strength steel, CP1180HD. Uddeholm Vanadis 8 SuperClean containing only a carbide phase and Uddeholm Vancron 40 containing a mixture of carbides and carbonitrides were also tested to compare the performance of the tool steels. The microstructure and wear mechanisms were characterized with scanning electron microscopy. It was found that the carbonitrides presented in Uddeholm Vancron SuperClean improved its resistance to material transfer and galling. Semi-industrial punching tests also confirmed that Uddeholm Vancron SuperClean cold work tool steel also possesses enhanced resistance to chipping and fatigue crack nucleation, which confirms the beneficial role of the carbonitride phase in wear resistance of cold work tool steel.

Solid-state NMR investigations of the polymer route to SiBCN ceramics

Silicon based polymers obtained by ammonolysis of organochlorosilylboranes and their pyrolytic transformation into Si-B-C-N ceramics were studied by a detailed solid-state NMR investigation. Sol–gel polymerisation/pyrolysis routes were applied to form Si-B-C-N materials with exceptional high-temperature stability. The polymer to ceramic conversion was analyzed by 11B, 13C, 15N, and 29Si MAS NMR spectroscopy as well as by thermal analysis measurements coupled with mass spectroscopy (TGA–MS). The results showed that a significant change in the carbon-, silicon-, and boron-coordination environments occurs during pyrolysis. An evolution of cleavage of silcon–carbon–boron bridges and the formation of new BN3 sites was observed. The NMR data obtained suggest the presence of a rather homogeneous dispersion of the boron atoms in the as synthesized silicon carbonitride phase, supporting the high thermal stability with respect to decomposition found in these compounds.Key words: organosilicon polymers, polymer pyrolysis, SiBCN ceramics, solid-state NMR.

Experimental realization of diamond-like carbonitride

Recently Cohen has predicted, based on an empirical model, that tetrahedral solids of low ionicity with high bulk moduli approaching that of diamond may be candidates as new hard materials. Several groups have attempted to synthesize these covalent C-N solids using vacuum deposition techniques. Although many of the literature results give C-N compounds with amorphous structures, two groups have reported the presence of a few percent of a crystalline phase, the so-called β-C3N4, after β-Si3N4 (hex.) structure, in an otherwise amorphous matrix. It has generally been difficult to synthesize samples with higher crystalline phase content and to confirm these early results. In samples prepared by a chemical precursor route, we report the presence of a crystalline carbonitride phase with zinc blende structure from electron diffraction and spectroscopy investigation. Based on a scaling relationship from Cohen, we estimate the bulk modulus of the new crystalline compound (denoted as α-C4N4 heretofore) to be comparable to c-BN and diamond from the C - N bond length determined from extended energy loss fine structure analysis, EXELFS, that allows a precise measurement of the first and second nearest neighbor distances between C and N atoms.

The mechanism of combustion synthesis of titanium carbonitride

Titanium carbonitride, TiC0.5N0.5, is synthesized directly by a self-propagating reaction between titanium and carbon in a nitrogen atmosphere. Complete conversion to the carbonitride phase is achieved with the addition of TiN as diluent and with a nitrogen pressure ≥0.6 MPa. Thermodynamic phase-stability calculations and experimental characterizations of quenched samples support a proposed mechanism in which the formation of the carbonitride is a two-step process. The first step involves the formation of the nonstoichiometric carbide, TiC0.5, and is followed by the formation of the product by the incorporation of nitrogen in the defect-structure carbide to form the carbonitride solid solution.