Superhard nanocomposites

Submitted By:Zaahir Salam

Submitted To:Dr. A. SubramaniaAssociate ProfessorCentre for Nanoscience and Technology Pondicherry University

Vickers hardness test The Vickers hardness test was developed in 1921 by Robert L. Smith and

George E. Sandland at Vickers Ltd as an alternative to the Brinell methodto measure the hardness of materials.

The Vickers test is often easier to use than other hardness tests since therequired calculations are independent of the size of the indenter, and theindenter can be used for all materials irrespective of hardness.

The basic principle, as with all common measures of hardness,is to observe the questioned material's ability to resist plasticdeformation from a standard source. The Vickers test can beused for all metals and has one of the widest scales amonghardness tests.

The unit of hardness given by the test is known as the VickersPyramid Number (HV) or Diamond Pyramid Hardness (DPH).

The hardness number can be converted into units of pascals.The hardness number is determined by the load over thesurface area of the indentation and not the area normal tothe force, and is therefore not a pressure.

To calculate Vickers hardness number using SI units one needs to convert the

force applied from kilogram-force to newtons by multiplying by 9.806 65

(standard gravity ) and convert mm to m

Introduction

Today’s mechanical systems are in need of continuousimprovements in enhancement of performance,durability and efficiency of the components.

So, Super hard nanocomposites have gained attentioninorder to satisfy the needs.

Highly sophisticated surface related properties such asmechanical, chemical and tribological properties ofsuper hard nanocomposites provide a best solution forthe improved efficiency of today’s mechanical systems.

Classification

On the basis of its hardness, nanocomposites are classified into 3 categories

1) Hard materials – Hardness greater than 20 Gpa

2) Super hard materials – Hardness greater than 40 Gpa

3) Ultra hard materials – Hardness greater than 80 GPa

Super Hard Nanocomposite

Nanocomposites are materials that comprises oftwo different materials in a standard proportion,whose properties are better than the individualmaterials.

Super hard nanocomposites (SHN) are thosewhich posses a vicker’s hardness greater than 40GPa.

Such materials are widely applied as coatingsover the mechanical devices.

Classification of Super Hard MaterialsMaterials

Generally, Super hard materials can be broadly classified into 2 types

Intrinsically super hard materials

Hardness arises due to the atomic arrangement itself. (E.g) Diamond, c- Boron Nitride.

Extrinsically super hard materials

Hardness arises due to the external processes such as ion bombardment, production of nanocomposites.

(E.g) nc-MN/a–Si3 N4 (M = Ti, W, V,etc.)

Intrinsically Super-Hard Materials

Diamond and cubic BN generally exhibits thissuper hard capacity.

This is due to the arrangement of the atomspresent in the structure. For example, indiamond, the super hard property is due to thediamond cubic structure of the crystal.

Hence, such materials found applications inmechanical systems, where high load and heatbearing capacity is inevitable.

Extrinsically Super- Hard Materials

In extrinsically super hard materials, thehardness is induced/generated in two ways

1. Ion bombardment

2. Formation of composites

Ion bombardment

The hardness of materials can be enhanced by bombarding the deposited film using high energy ions.

The hardness enhancement is due to a complex effect involving o decrease of the crystallite size

o densification of grain boundaries

o formation of frenkel pairs and other point defects

o built in biaxial compressive stress.

Hardness (vs) Compressive stress

• First, very high enhancementof the hardness of TiN (up to80GPa) and (TiAlV)N (up to100 GPa) during depositionby means of unbalancedmagnetron sputtering atnegative substrate bias isfound.

• Later it was found that thereis a correlation existbetween the hardnessenhancement and the biaxialcompressive stress inducedin the films.

Reason for Hardness

• The highest hardness enhancement upon energetic ionbombardment is obtained in refractory hard ceramic coatingsdeposited at a relatively low temperature of about <300oC.

• At a higher temperature, the hardness enhancement decreases andcompletely vanishes above 600–700 oC

• Reason: The ion-induced effects anneal out during the film growthwithin the deeper regions that are not accessible to the ions withtypical energy of a few 100 eV and corresponding projected rangesof <10 nm.

• When the compressive (or tensile) stress is induced in a bulkspecimen by bending it, such an enhancement (or decrease)doesn’t corresponds only to the amount of that stress.

• Therefore, a compressive stress alone can never enhance thehardness to 60–100 Gpa , then ??????

Other Factors

The hardness enhancementresults from a complexsynergistic effect of the

– decrease of crystallite size

– densification of grainboundaries

– built in compressive stress

– Formation of radiationdamage (Frenkel pairs,etc.) upon energetic ionbombardment

Super Hard Composites

Depending on the crystallite size, the above said factors mayhinder the dislocation activity

Dislocation activity is absent in the superhard thermally highlystable nanocomposites that consist of a few-nanometer smallcrystallites of a hard transition metal nitride (or carbide,boride,...) glued together by about one-monolayer-thin layerof nonmetallic, covalent nitride such as Si3N4, BN (or in thecase of carbides by excess carbon, CNx, and others)

Properties

• These coatings, when correctly prepared, posses anunusual combination of mechanical properties, such as

• high hardness of 40 to 100 Gpa

• high elastic recovery of 80% to 94%,

• elastic strain limit of 10%, and

• high tensile strength of 10 to 40 Gpa

• Moreover, the nanostructure and the superhardness(measured at room temperature after each annealingstep) remain stable up to 1100oC

Example

• Superhard “Ti–Si–N” coatings were producedby means of plasma induced CVD (P CVD) usingchlorides as a source of Ti and Si.

• It is attributed the hardness enhancement tothe precipitation of small Si3N4 particles withinTiN nanocrystals.

• The maximum hardness of 60–70 Gpa wasprobably due to ternary nc-TiN/a-Si3N4/a-TiSi2nature of this coatings.

Hardness - Composite formation• In the majority of coatings deposited by PVD techniques at low

pressure of the order of <10-3 mbar and negative substrate bias,there is a large biaxial compressive stress of 5 – 8 GPa due tothe energetic ion bombardment during their deposition.

• To check “if the measured hardness is not only enhanced bythat bombardment”

• When the coatings deposited were annealed to 600–800oC, thehardness increased to about 40 Gpa and the originallyamorphous films showed nanocrystalline XRD patterns.

• Thus, although the TiB2.2and TiN coatings deposited have a highhardness enhanced by energetic ion bombardment during theirdeposition , the hardness of the “Ti–B–N” coatings from themiddle of the nitrogen range in is due predominantly to theformation of the nanocomposite structure

• The hardness maximum at about 27% of nitrogen is lesspronounced than usually found in our nanocomposites depositedby P CVD.

Evidence

Ion bombardment (vs)Composites

Stability of the hardness upon annealing

• The hardness that has been enhanced by energeticion bombardment strongly decreases withannealing temperature to the ordinary bulk valueupon annealing to 400–600oC

• Superhard nanocomposites remains unchangedupon annealing up to 1100oC .

• This softening upon annealing of the superhardcoatings hardened by energetic ion bombardmentis a general phenomena associated with therelaxation of ion-induced defects in the films thatcauses the hardness enhancement duringdeposition.

Stability of Hardness with annealing

Ion bombardment (vs)Composites

Dependence of hardness with composition.• TiN1-xCx forms a solid solution and therefore thehardness follows the rule-of-mixtures• In the case of the so-called nanocomposites,consisting of a hard transition metal nitride andductile metal, the maximum hardness is achievedwith the pure nitride without that metal.• The superhard nanocomposites preparedaccording to our design principle show amaximum hardness at a percolation threshold.

Hardness (vs) Composition

Intrinsic vs Extrinsic

Extrinsic materials are far better than the intrinsic materials.

Hardness (vs) Grain size

• With a decrease in the grain size, the hardness of thematerials increases.• Hall-petch relationship

H(d) = H0 + Kd-1/2

• Dislocation movement, which determines the hardnessand strength in bulk materials, has little effect when thegrain size is less than approximately 10nm.• At this grain size, further reduction in grain size bringsabout a decrease in strength because of grain boundarysliding.

Hall petch relationship

Hardness Measurement

• Good mechanical properties of a coating require•High hardness,•High toughness•low friction•High adhesion strength on substrate•Good load support capability and•Chemical and thermal stability, etc.

• At present, nanoindentation is regarded as a goodmethod in hardness determination.• In nanoindentation test, a diamond indenter is forcedinto the coating surface. The load and depth ofpenetration is recorded from which the hardness andother elastic properties are calculated.

Applications

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