Nanomechanical Test Instruments - A Buyers Guide

Mohan Ranganathan, Professor, Department of Mechanics and Systems, Polytechnics Engineering School, University of Tours, Tours, 37200, France
Corresponding author: ranganathan@univ-tours.fr

Introduction
What do I want to measure?
What materials do I want to work with?
What are my main application areas?
What level of accuracy do I need?
Who will use the instrument?
Am I aiming to publish data from the instrument?

Are technical specifications such as load resolution etc important?

What consumables will the instrument use?
What will be the lifetime support cost of the instrument?

Can the manufacturer provide published data to support its claims?
Can the manufacturer provide references from reputable institutes?
Is there local service and support provision?

Introduction

Nanomechanical test methods such as nanoindentation and nano-scratch are now well established techniques for the characterisation and optimisation of thin films, coatings and micro-scale structures. There are a number of manufacturers, offering many variations on the techniques, and when it comes to purchasing an instrument, it’s difficult to know what evaluation criteria to use. The following are some points for consideration, which may act as a guide to help you to make the best choice for the research you wish to carry out.

Technique Information Given
Nanoindentation Hardness, Modulus, Creep behaviour
Nano-scratch & wear Critical loads, scratch resistance (time or cycles to failure), friction information.
Nano-impact & fatigue Fracture and fatigue behaviour, damping capability, energy dissipation dynamic hardness
Nano-fretting Wear under reciprocating conditions, cycles and time to failure

What do I want to measure?

This may seem an obvious question, but it’s extremely important to consider what information you need from your system. Nanoindentation is perhaps the most common of all the nanomechanical testing techniques, but will that alone give you the information you seek?

Nanoindentation can be further split into quasi-static (sometimes referred to as ‘Depth Sensing Indentation or Instrumented Indentation Testing) and dynamic methods. Quasi-static indentation allows the material time to react to the force being applied to it. Dynamic indentation uses high strain rates to assess strain rate sensitivity. Additional capability in the form of oscillatory techniques may be offered. These have been used in the past to assess storage and loss moduli of materials, in particular polymers. The accuracy and usefulness of these techniques have in recent years been called into question and as such these techniques have fallen out of favour.

What materials do I want to work with?

Different materials will require different priorities in your system choice. Soft materials such as biomaterials and polymers will require quite low loads (as low as 30ìN) and as such your load resolution will be important. For much harder materials such as wear resistant coatings, force resolution - while still of importance - will be less of an issue. The loads required for coatings such as this (up to 500mN) require a robust and accurate loading system. Thicker coatings and functionally graded materials will need a wide load range.

If the system is to span a range of materials from the very hard to the very soft, then the instrument should demonstrate both of the above traits. Some manufacturers offer a dynamic resolution system, allowing them to span the full load range (30ìN to 500mN) with no loss of resolution at the lower end of the load range.

What are my main application areas?

It is extremely important that your materials characterisation or optimisation programme mimics the real-life working conditions your material will see in service. Many materials will only work in ambient conditions, so the conditions supplied by the average indentation system will suffice. However, many materials will work in a variety of conditions, such as high strain rate, at high or low temperature, in inert gases, under different humidity’s or under liquids. Mechanical properties will change greatly under each of these conditions, so for optimum results testing under these conditions is extremely useful.

What level of accuracy do I need?

If the instrument is to be used as a QA tool then consistency rather than accuracy is required. However, if this to be a dedicated R&D instrument, both consistency and accuracy are key. Key aspects to consider:

Instrument drift: Instrument drift has two main components – electronic drift and thermal drift. Most high end systems use excellent electronics, meaning thermal drift is normally the main difference between their performance. Instrument drift is normally quoted in nm/s. Due to the small depths being measured with these nanoscale techniques this figure needs to be as low as possible, certainly below 0.005nm/s. Manufacturer’s should be asked to demonstrate the drift of their instrument over a set time period using fused silica, an accepted reference sample.

Who will use the instrument?

Is the instrument to be used by one specialist group, or will it be in a central facility where it is available to a number of users? What level of expertise will the users have? Whatever the answer, the system must be easy to use, while offering some flexibility of experiment design. Ideally a system demonstration (on the current model) should be available prior to purchase to assess this.

Am I aiming to publish data from the instrument?

Nanoindentation as a technique is well established, and a routine measurement for the assessment of coatings and thin films. Therefore while still very relevant, such measurements are no longer cutting edge. Therefore, if you can display some differentiation or innovation in your research, publications will perhaps be more readily accepted. Combinations of techniques, such as high temperature indentation, or indentation and impact data, will give an edge to your research as it will present a much fuller picture of your material performance, and give the community something new.

Are technical specifications such as load resolution etc important?

Absolutely, however they are not the most important thing. When measuring on this scale, resolution is key. However, other aspects such as thermal drift may render a load resolution figure meaningless. Noise floors are another such specification. Manufacturer’s tend to quote theoretical specifications, so it’s more meaningful to see what they can actually achieve in practice.

Again, while accuracy and resolution are of course important, papers aren’t published on the strength of accuracy of load application. Therefore it’s important to be led by technique or application rather than by a comparison of specifications.

What consumables will the instrument use?

Lifetime cost of the instrument is important, and so it is useful to get an idea of this before purchasing. Ask your manufacturer for a price list for their indenters, and any consumables the system may need in normal operation.

What will be the lifetime support cost of the instrument?

Will your manufacturer charge you for email and phone support? Will they charge for software updates or provide these free of charge?

Can the manufacturer provide published data to support its claims?

If you are unsure of any claims made by any manufacturer, ask to see a publication to validate the claim. This should be made available. Beware of those who can’t supply such data.

Can the manufacturer provide references from reputable institutes?

References should be sought on three areas: Performance of the instrument, quality of service and support, and ease of interaction with the manufacturer. All three of these items will be of importance to you and your research.

Is there local service and support provision?

Ideally a manufacturer-trained engineer should be in your country for quick replies to support issues. If not, ask the manufacturer which country support will be provided from.

Date Added: May 2, 2011 | Updated: Jun 11, 2013
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