GB2348279A

GB2348279A - Coating thickness measurement by remote non-contact photothermal method

Info

Publication number: GB2348279A
Application number: GB9906399A
Authority: GB
Inventors: Peter Edmond Chandler; Michael Bernard Coupla Quigley
Original assignee: C A TECHNOLOGY Ltd
Current assignee: C A TECHNOLOGY Ltd
Priority date: 1999-03-20
Filing date: 1999-03-20
Publication date: 2000-09-27
Also published as: GB9906399D0

Abstract

The thickness of a coating 9 applied to a surface 10 for protection against corrosion or erosion is measured using a photothermal technique in which a modulated beam from a source 2 is directed onto the coating to cause a thermal wave to pass through the coating and the coating/substrate interface into the substrate. When the thermal wave hits the interface it is reflected and flows through the coating to be emitted as thermal signal with the same modulation as the incident beam but delayed in phase. The emitted radiation is discriminated from the incident radiation by its longer wavelength and is detected by a detector 11 connected to a phase analyser 13 such as a lock-in amplifier. Thicker coatings give longer phase delays and the thickness values are determined using reference standards. A conventional light source 1 with filtering 7 is used in place of a more expensive and hazardous laser.

Description

Patent Application This invention refers to the measurement of the thickness of coatings applied by whatever means to a surface to impart protection against corrosion or erosion or for other engineering or decorative functions.

For example MCrAIY overlay coatings are applied to gas turbine blades and vanes in order to provide resistance to high temperature sulphidation and oxidation. For example paint coatings are applied to automotive parts to provide corrosion protection and decoration.

There have been a number of developments and patents on this subject particularly with regard to this measurement taking place without contact between the measuring instrument and the surface of the coating. Almond et al have written a number of papers on the subject of non contact measurement using a photothermal technique.

The principe of the Photothermal technique for the measurement of coating thickness is that a modulated laser beam is caused to impinge on the surface of the coating to be measured and the subsequent thermal wave passes through the coating and the coatingsubstrate interface and into the substrate. When this thermal wave hits the coatingsubstrate interface part of this thermal wave is effectively reflected and flows through the coating to be retransmitted as another thermal signal. The wavelength of this emitted signal is longer than that of the incident wave and in that way they can be discriminated.

The emitted signal is detected by a sensor; it is also modulated as is the incident signal but it has a phase delay which can be measured by a phase sensitive detector such as a lock-in amplifier. Essentially the greater the coating thickness the greater is the phase difference and hence the coating thickness may be determined by reference to previously obtained reference standards for that particular coating material.

The incident beam is generated by a laser source, typically a laser diode, which may be operating at a frequency of 828nm for example. Light reflected from this single frequency source at the surface of the coating is prevented from reaching the detector by means of interference filters which act as a barrier to that wavelength but pass the higher wavelengths radiated from the surface. ~~~~ Our new invention specifically refers to the replacement of the laser light source with altemative light sources. The laser is a highly directional coherent light source with a very high brightness at its specific wavelength. The wavelength is chosen so that it does not interfere with the emitted light from thé surface. The highly directional aspects and the coherence of the laser are of no benefit this measurement. Also, and very importantly, the laser has a safety problem. Due to the coherence and directionality of the light from the laser it represents a hazard which is well established and documented. Essentially the laser can represent a hazard unless great precautions are taken to prevent accidental exposure of the human eye to the ? er beam. This may involve the use of interlocks around the measuring system and witfprobabty involve the article being measured being placed inside such a system. If the article to be measured is physically large then this represents a significant problem and one which has prevented the widespread application of this technology.

Our invention allows for the replacement of the laser as the light source by a conventional light source with the addition of filtering systems. The principle is that the conventional light source emits light over a number of wavelengths in the visible and near infrared range. If an interference filter is used to eliminate the near infra red and all radiation at wavelengths beyond a certain wavelength then it can be effective as the light/heat source for this application. Although a significant proportion of the power output of the light source is blocked by the interference filter the starting power can be as high as necessary so as to provide adequate power to the surface of the coating. For example if the laser power output is, say, 0.5 watts it may be necessary to provide a conventional light source of, say, 20 watts or more. After filtration the effective power being transferred to the surface may be comparable with the laser source. This is not a problem as the conventional light source is so inexpensive, especially compared with the cost of the laser system.

A further advantage of the proposed invention is that it can be used as an on-line measuring system during the coating process. At present the component being coated needs to be measured when the coating equipment is off line. In the case of some coating techniques, namely thermal spraying processes which have to be carried out inside a closed chamber, it is very time consuming and not cost effective for the coating equipment to be closed down for the thickness measurement to be carried out. Being able to do the measurement on line will have very significant quality control and economic advantages.

A further advantage is that it is possible to carry out these measurements remote from the item being measured. This will have particular advantages when the item in question is physically difficult to access, or is radioactive or, as in the case of paint, is still drying whilst the measurement is carried out.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which Figure 1 shows the optical and electrical circuitry. A light source 1 containing its own filtration to reduce the infra-red emissions is connected to a power supply 2. The light beam 3 from the light source 1 is then transmitted through a fiber optic 4. A light chopper 5 whose frequency of rotation is in turn controlled by a power supply 6 provides the modulation to the light beam 3. It may be appropriate to insert an optical filter 7 to reduce the infra-red emissions from the light source. A lens 8 focusses the light beam onto the coated surface 9.

The emitted signal from the surface 9 of the coating 10 is focussed by reflecting or refracting optics onto a detector 11. The electrical signal 12 from the detector 11 is then passed into a comparator or phase analyser 13 which compares the phase of the emitted signal with that of the incident signal as measured by an optocoupler 14. The output of the phase analyser 13 is displayed on an indicator 15.

Claims

What we claim is 1. A device for the measurement of coating thickness using the photothermal technique whereby a modulated light beam is projected onto the surface of the coating to be measured, the thermal wave reflected back from the interface between the coating and the substrate is detected from the surface of the coating and the phase difference between the two signals is measured thus indicating coating thickness.
2. A device as claimed in claim 1 wherein the light source is generated by a non-laser source with the addition of filtering systems to produce the appropriate wavelengths.
3. A device as claimed in claim 2 wherein the light source of claim 2 is modulated by a mechanical light chopper with varied shapes to produce the modulation waveform desired.
4. A device as claimed in claim 2 wherein the device may be situated at a distance of greater than 100 mm from the surface being measured.