Influence of the processing route on the properties of Ti(C,N)-Fe15Ni cermets

https://doi.org/10.1016/j.ijrmhm.2019.105046Get rights and content

Highlights

  • Two routes were used for preparation of powders in the system Ti(C,N)-Fe15Ni: colloidal dispersion and wet milling.

  • The processing route directly affected the C content of the mixture powders.

  • A higher quantity of C in the final sintered materials favoured their density and mechanical integrity.

  • Small-scale hardness and sliding contact tests revealed influence of processing route on mechanical properties.

Abstract

This study aims to understand the influence of powder preparation and processing steps on the microstructure and properties of Ti(C,N)-Fe15Ni cermets with 70 and 80 vol% of ceramic phase. Two routes were used for powder preparation: (i) a colloidal approach, consisting of the preparation of stable aqueous suspensions of the powder particles and spray-drying to obtain easy-to-press granules, and (ii) conventional powder metallurgy route, consisting on wet ball milling of powders, with subsequent drying in rotary evaporator. The resultant powder mixtures were uniaxially pressed and sintered in high-vacuum at 1450 °C for 2 h. Sintered samples were characterized in terms of their density, porosity, microstructure (FESEM, image analysis), composition (EDX and XRD), small-scale hardness and sliding contact response by means of massive nanoindentation and nanoscratch testing. C content of the mixture powders was lower for conventional route, lost during milling. After sintering, all the materials, despite the processing route and composition, show C reduction, although that outflow is higher for the conventional powder metallurgy route, and more evident for the composition with higher binder content. As a result, COL samples exhibit a more homogeneous microstructural assemblage, higher small-scale hardness and mechanical integrity under sliding contact conditions. Compositions of materials must then be adjusted to adequate initial C addition with respect to the employed processing route, to account for the effects of the mixtures preparation stage.

Introduction

The cutting tool industry is actively searching for new materials as substitutes for traditional WC-Co cemented carbides, due to the inclusion of their constitutive elements in European and American lists of toxicity-related and critical raw materials [1,2]. Titanium carbonitride, Ti(C,N), arises as a feasible ceramic competitor, as it features high hardness, wear and corrosion resistance, and chemical stability [3,4]. Regarding the substitution of the cobalt-based binder, iron stands as an excellent candidate, having a relative low price and being non-hazardous for the human health [5]. Several investigations have claimed the advantage of using Fe instead of Co binders, for example, by improving the oxidation resistance of the final materials [[6], [7], [8], [9]]. Nevertheless, the combination of Fe and Ti(C,N) has reported wettability problems [10], which directly translates into poor sinterability when processed by powder metallurgy.

Avoiding the addition of secondary carbides that could improve above sintering issue [11,12], another strategy to overcome lack of wettability for Fe-Ti(C,N) system is related to addition of binder alloying elements. Nickel has demonstrated throughout the years excellent capability to attain homogeneous and dense sintered hard materials [[13], [14], [15]]. Previous investigations demonstrated that introducing 15 wt% of nickel as alloying element in the binder phase can improve binder-ceramic wettability, and lead to cermets with final properties comparable to some cemented carbides [10,16]. However, these materials are very sensitive to the processing route [17,18].

Following this, the present study aims to understand the influence of the powder preparation and processing steps in the microstructure and properties of Ti(C,N)-Fe15Ni cermets, using two ceramic contents: 70 and 80 vol%. In doing so, extra carbon (C) was added to enhance the sinterability of the materials, as it is an element known to lower the solidus and liquidus temperatures of Fe [10,19]. Two routes were used for powder preparation:

  • Colloidal (COL) route – consisting of the preparation of stable aqueous suspensions of the ceramic-metal powder particles and spray-drying to obtain easy-to-press granules, with a characteristic raspberry shape.

  • Conventional powder metallurgy route (CPM) – entailing the ball milling of the metallic and ceramic powders.

Resulting powder mixtures were uniaxially pressed and sintered in a high-vacuum furnace. The final sintered samples were characterized in terms of their density, microstructure, composition, nanohardness and sliding contact response at small length scale. The final objective was to reveal the influence of the processing route in the C content of powders and sintered samples, a parameter that directly influences the final properties.

Section snippets

Experimental procedure

The compositions used in this investigation are detailed in Table 1, correlating the volume of ceramic/metallic phases employed with reference-like WC-Co cemented carbide grades (Co wt%). For both material configurations, 0.5  wt% of extra carbon, with respect to the metallic binder content, was added.

Elemental powders were mixed and prepared following colloidal and conventional powder metallurgy routes, respectively. In the COL route, stable aqueous suspensions of the ceramic and metallic

Carbon content

Table 2 collects the C content measurements of the cermet powders after COL and CPM processing, as well as that of resultant samples after the sintering process. First, it is noticeable that CPM mixture powders contain less C than colloidal ones for both compositions, a difference that is slightly lower for 80TiCN composition. A hypothesis for this C reduction is the reaction of this element with oxygen or its adhesion to the mill vessel and balls during the 24 h tumbling milling. Moreover, two

Conclusions

In this investigation, Fe15Ni-TiCN cermets were manufactured, employing two processing routes – colloidal (COL) and conventional powder metallurgy (CPM) – and two compositions, 70 and 80 vol% of ceramic phase. The following conclusions could be drawn:

  • For the same ceramic/metal composition and carbon addition, it could be stated that CPM powder mixtures lost C during milling, due to reaction with oxygen and adhesion to milling balls/vessel.

  • Sintered materials showed C loss, despite the processing

Acknowledgements

The authors would like to acknowledge the financial support from the Ministerio de Economía y Competitividad, Spanish through the project MAT2015-70780-C4-P and grant BES-2016-077340, and the Regional Government of Madrid through the program ADITIMAT, Ref. S2018/NMT-4411. J. J. Roa acknowledges the Serra Hunter programme of the Generalitat de Catalunya.

References (32)

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