STRUCTURE AND PROPERTIES OF BORON NITRIDE BASED COMPOSITE PRODUCED BY SHS METHOD

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STRUCTURE AND PROPERTIES OF BORON NITRIDE BASED
COMPOSITE - PRODUCED BY SHS METHOD

• Lembit Kommel, Jakob Kybarsepp, Irina
  Hussainova, and Eduard Kimmari
• Department of Materials Engineering
• Tallinn University of Technology
• Ehitajate tee 5
• P.O. Box 19086
• Tallinn, Estonia
• Fax [phone]
• Phone [phone]
• kommel_at_edu.ttu.ee

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ABSTRACT

• Lightweight cubic boron nitride (c-BN) based
  composite with addition of Ti, Fe, Cr, Ni and C
  as a binding phase was studied.
• This material was produced by a SHS method
  followed by hot compaction.
• A second hard phase formed within a steel binder
  phase was found.
• Those titanium carbide (TiC) spherical particles
  were up to 600 nm in diameter and uniformly
  distributed over a metallic phase. However, an
  interface between large c-BN grains and a
  stainless steel of FeCr25Ni11 binder was
  saturated with titanium, nitrogen, carbon and
  nickel.
• Vickers hardness of the material was 850 HV10.
• Tribological properties of the material were
  investigated under conditions of dry sliding and
  hydroerosion in sodium solution slurry.
• It was shown that c-BN based composite has a
  higher wear resistance as compared to WC-CoNi
  hard metals, TiC-Ni cermets and B4C-Al composites.

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Material manufacturing features

• To produce a material, the elemental c-BN powder
  of the average particle size of 20 ?m was used.
  At the same time this powder mixture contains
  small amount of the very large c-BN particles of
  50 ?m in a cross-section.
• The initial powder consisted of c-BN-25, Ti-20,
  Fe-29.5, Cr-11.5, Ni-9, and C-5 (in wt. ) and
  had been mixed in a planetary mixing machine
  during 5 h.
• Then the powder was closed into a steel capsule.
  To start an SHS-process, the capsule was heated
  up to temperature of 1150 C. The combustion
  temperature was increased up to 1250-1300C with
  a low self-propagation rate. Immediately after
  SHS-process the heated capsule was subjected to a
  hammer forging.
• For testing the laminas of the prepared composite
  were sectioned, diamond polished and
  ultrasonically cleaned.

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Material testing methods

• Composite microstructure was studied with the
  scanning electron microscope (SEM) Gemini LEO
  Supra-35.
• Hardness of composite was measured with the
  Vickers hardness tester Indentec and
  microhardness of phases was measured with the
  microhardness tester Micromet-2001, Buehler. Time
  of the loading (during microhardness testing) was
  12.5 s and load was 10, 25 and 50 g depending on
  components hardness, brittleness or measurable
  variable.
• Mechanical and physical properties of composite
  were investigated using the microindentation
  method based on the Zwick Z2.5/TS1S installation.
  
• Wear rate and friction coefficient in dry sliding
  condition were measured according to ASTM-B
  611-85. Test parameters were as following
  sliding distance up to 12 km normal test load of
  40, 180, 220 and 320 N system of steel
  ringcomposite and linear velocity of 2.2 m/s.
• Slurry eroded sample weight loss had been defined
  during 144 hours in sodium solution media.

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Structure of c-BN based composite
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Two ceramic phases c-BN and TiC in stainless
steel binder
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C-BN Grain in White Color
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Chemical elements distributions
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Phases content of composite
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Chemical Elements Distribution in Composite
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Elements Distribution in Binder Phase
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Coalescence of nano size TiC spherical particles
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Mechanical and physical properties

• Vickers hardness number measured with
  application of load of 10 kg is 850 HV10.
• Because of small size (about 600 nm) of the
  second hard phase, measurement microhardness of
  TiC particles was not possible on Mikromet 2001.
• The microhardness of composite has been
  determined applying different loads 10, 25, and
  50 g during 12.5 s of indentation time.
• Microhardness of binder phase is 1465 HV 0.05 and
  micro-hardness of the soft dross inclusions is
  about 900 HV 0.05.
• The large c-BN grains have a very high
  micro-hardness of 7000 HV 0.05 that is revealed
  with diamond microhardness.
• The universal hardness of composite is HU
  100/7.5/15 5500 N/mm2 and the plastic part of
  universal hardness is HU plast 100/7.5/15 8500
  N/mm2.
• The indentation module (module of elasticity) of
  composite is Y HU 100 190 kN/mm2 that is
  revealed with hardened steel module of
  elasticity.
• The mean ratio of elasticity ?HU 28.
• The composite has relaxation of RHU 2.15 and
  creep of CHU 2.18 .

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Wear coefficient by dry sliding

• Effect of normal load on the wear coefficient
  curve K1 first series and curve K2 second
  series.
• Figure represents the calculated wear coefficient
  in dependence on normal load. It can be supposed
  that only the large BN particles contact with
  steel ring while sliding under a load of 40 N.
• An increase in the load causes breaking of the
  large BN grains and an increase in wear rate.
• The composite has two hard phases, micro size
  c-BN and nano size TiC in stainless steel binder.
  
• At the first series the wear rate is higher
  (curve K1) than at the second test series (curve
  K2).

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Results of Slurry Erosion and Wear Resistance
Testing

• Weight loss by slurry erosion in sodium solution
  during 144 h is only 0.1 mg/h.
• The composite stainless steel binder, hard phase
  c-BN and second hard phase TiC have a very high
  oxidation resistance 1-6.
• The c-BN based composite has a higher wear
  resistance by slurry erosion as compared to
  WC-CoNi hard metals, TiC-Ni cermets and B4C/Al
  composites.
• The friction coefficient of composite by try
  sliding over the steel ring is 0.23 and the wear
  rate is 0.091 mm3/km. These data have been
  obtained on distance of 8 km under normal load of
  150 N. Low friction coefficients and low wear
  rate under relatively low normal load point that
  friction can occur only between the large c-BN
  grains and a counter body.
• Increase in a normal load leads to a proportional
  increase in wear rate.

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Conclusions

• The new nano sized second hard phase in stainless
  steel metallic binder has been sintered during
  SHS process.
• The wear coefficient decreases by normal load
  increase when the TiC nano size spherical
  particles involved in wear and the tribofilm
  formation took place, and the friction mechanism
  change as result.
• The relatively low hydroerosion wear of the
  composite in sodium solution slurry means that
  the all compounds have high erosion resistance.
• The c-BN based composites may be filling an
  important role as one of the key advanced
  materials for modern technology in applications
  where light-weight, hard, oxidation and wear
  resistant materials are necessary.

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• Thank you for the attention!