In Situ Monitoring of Particle Consolidation During Low ... - Springer Link

JTTEE5 20:845–851 DOI: 10.1007/s11666-011-9631-3 1059-9630/$19.00 ASM International

R.Gr. Maev, S. Titov, V. Leshchynsky, D. Dzhurinskiy, and M. Lubrick (Submitted November 22, 2010; in revised form December 27, 2010) This study attempts to test the viability of the examination of the cold spray process using acoustic methods, specifically in situ testing during the actual spray process itself. Multiple composites studied by flat and multi-channel transducers as well as the results of actual online measurements are presented. It is shown that the final thickness as well as the dynamics of buildup can be evaluated (including plotting rates of buildup). Cross sections of the coating thickness are also easy to obtain and show true profiles of the coating. The data can also be used to generate real estimates for nozzle speed and spray diameter. Finally, comparisons of real thickness and acoustically estimated thickness show a close linear relationship. The data clearly show that online acoustic measurement is a viable method for estimating thickness buildup.

Keywords

build-up process, coating, cold spray, monitoring, ultrasonic pulse-echo method

1. Introduction Cold spray (CS) is a rapidly developing coating and manufacturing technology in which the spray particles (in the range of 10-150 lm) are fed into a supersonic gas stream accelerated to a high velocity and deposited on a substrate in solid state (Ref 1-6). The severe plastic deformation of the particles on impact produces a deposit that is very dense, with low oxide content and no thermally induced tensile stresses. A typical feature of CS is the relatively low temperature of driving gas compared with the conventional thermal spray techniques. Accordingly, the temperature of spray particles prior to impact is much lower than their melting point. Other important advantages, especially when compared to thermal spraying techniques, are the low oxygen content within the material, the relatively high deposition rates (10-30 kg/h) achievable and the compressive residual stresses after deposition as solidification shrinkage and shrinkage on cooling are avoided. The low temperature characteristic of CS makes it possible to deposit the coating of the materials without any significant change in the microstructure of feedstock. Most metals, such as Cu, Al, Ti, and their alloys, have been deposited by CS (Ref 6-8), and even

R.Gr. Maev, Department of Physics, University of Windsor, Windsor, ON, Canada; and R.Gr. Maev, S. Titov, V. Leshchynsky, D. Dzhurinskiy, and M. Lubrick, Institute for Diagnostic Imaging Research, University of Windsor, Windsor, ON, Canada. Contact e-mail: [email protected].

Journal of Thermal Spray Technology

cermets (Ref 9) and ceramic particles (Ref 2, 10) could be embedded into a metallic substrate to form a thin layer coating. It is important to understand the kinetics of cold spray coating formation, which can be roughly classified into three main processes: particle heating and acceleration, single and multi-particle impact accompanying particle deformation, and coating formation by particle consolidation. Based on the modeling work of particle acceleration (Ref 5, 6) and in-flight particle diagnostics (Ref 8), the upstream processes are now well understood. On the contrary, the subsequent two phenomena, i.e., particles impact and shock particle consolidation during the coating formation, have not been understood completely, especially due to the experimental difficulty. As a result, the process parameters could be related to the coating geometry and properties (e.g., adhesion strength) only by specific experiments. Although the experimental method itself is a strong tool for optimizing coating geometry and properties, recent development in on-line control of coating geometry and its main properties enables us to optimize the cold spray technology parameters within the limitation of low pressure cold spray processes. If we would elucidate the particle consolidation and kinetics of coating build-up, such an approach will then give new insight into cold spray technologies. A glance at the recent literature on different coating processes shows increased interest in depositing composite coatings with precise control of a build-up process. Being relatively new, there is not a lot of data describing the build-up process, especially for the low pressure case focused on by our group. In fact, other than previous papers by the authors (Ref 11), there seem to be no acoustic measurements of any type. Acoustic studies allow for the non-destructive determination of properties especially if models are developed to relate acoustic and structural properties. Even more important, this article

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In Situ Monitoring of Particle Consolidation During Low Pressure Cold Spray by Ultrasonic Techniques

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discusses the development of online monitoring (during spraying) of the process using acoustics methods. The focus is on the thickness build-up of the coating and demonstrating the feasibility of monitoring this acoustically.

The first pulse A is produced by the reflection of the ultrasonic wave at the interface between delay line and substrate (Fig. 1). The propagation time of this pulse is tA ¼

2. Procedure

2b ; C0

ðEq 1Þ

where C0 is the velocity of ultrasound wave in the acoustic delay line and b is its thickness. The pulse B is reflected

2.1 Materials and Spraying Commercially available aluminum (Al), alumina (Al2O3), and zinc (Zn) powders of particle size 325 mesh were mixed in the amounts shown in Table 1 (hereafter referred to by their names in the table). The composites were then applied using a portable apparatus equipped with an SST Centerline gun (Ref 7). This system utilizes the injection of powder into the divergent part of a supersonic nozzle. The powder mixtures were supplied by a powder hopper and were injected into the supersonic portion of the nozzle near the throat area by means of a negative pressure developed by an accelerated stream of compressed air passing through the nozzle. The injected particles are accelerated in the high velocity air stream by the drag effect. To increase the air velocity and, ultimately the particle velocity, the compressed air can be preheated within a range from 100 to 500 C. The pressure and temperature of the compressed air were monitored by a pressure gauge and a thermocouple positioned inside the gun. The gun was installed on an X-Y manipulator to scan the air-powder jet over the substrate surface.

Fig. 1 Scheme of the online ultrasonic cold spray monitoring system

2.2 Monitoring System In this article, we proposed to characterize kinetics of coating formation by means of ultrasonic pulse-echo method. The scheme of the ultrasonic online CS monitoring system is presented in Fig. 1 and 2. The spraying nozzle is translated along axis X with velocity V over the top surface of the substrate creating a layer of the deposited material. The process can be characterized by profile function h(x,y,T), which is the thickness of the material expressed as a function of time T, spatial longitudinal coordinate x, and transverse coordinate y. The ultrasonic probe is attached to the opposite surface of the substrate. It sends ultrasonic waves through a coupling media and the substrate to the area where the spray process takes place. Immersion liquid, such as water or hard delay line wetted with ultrasonic gel, can be used as a coupling media. The reflected ultrasonic signal received by the same transducer consists of several typical responses.

Fig. 2 Arrangement of the ultrasonic transducers (bottom view)

Table 1 Materials used Composite designation Al AlAlAlAl-

Particle designation

Average particle size, lm

Volume fraction of reinforcement, %

Substrate

… Al2O3 Al2O3 Al2O3 Al2O3

45 10 45 45 45

0 10 15 30 50

Steel Steel Al Steel Al

Al2O3-1 Al2O3-2 Al2O3-3 Al2O3-4

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tB ¼

2d ; CS

ðEq 2Þ

where d and CS are the substrate thickness and its wave velocity. If the coating is being sprayed on top of the substrate, the ultrasound wave partially penetrates into the deposited material and the wave being reflected from the top surface produces pulse C. The time delay s of the pulse C relative to pulse B depends on the thickness of the coating at a particular time of the process and position of the transducer: s¼

2hðx; y; TÞ Cm

ðEq 3Þ

Thus, it is possible to calculate the thickness of the material h(x,y,T) on the base of measurement of s and knowing the sound velocity in the deposited material Cm. Furthermore, by processing these data, it is possible to estimate the ‘‘deposition rate of the process’’ a(x,y), which may be defined as the speed of the thickness increase at a particular point if the position of the nozzle is unchangeable. If the nozzle is moving along x axis with velocity V and passes over the observation point (x,y), the thickness of the coating is simply an integral of the deposition rate: hðx; y; T Þ ¼

ZT

aðx V T; yÞ dT

ðEq 4Þ

0

So, using this equation, the deposition rate of the process a(x,y) can be estimated by differentiating of the measured thickness h over time T: aðx V T; yÞ ¼

dhðx; y; T Þ Cm ds ¼ dT 2 dT

ðEq 5Þ

To achieve sufficient lateral resolution in the transverse direction (along y-axis), it is convenient to have an array probe, whose elements are positioned perpendicular to the movement of the nozzle. In the experiment, we used an array probe consisting of 11 square elements. The elements are located in a zigzag manner as shown in Fig. 2. The size of the element is 1.2 mm, the pitch of the array in one row is p = 1.768 mm. The second row of elements reduces the effective pitch of the array to the value of p/2 = 0.884 mm. Due to the shift of the elements 2, 4, 6, 8, and 10 along x-axis relative to the first row at the distance Dx = p/2, the signals recorded by these elements have an additional time delay DT = p/(2 V). In the experiments, the velocity of the nozzle movement was V = 5 mm/s, therefore, DT = 0.177 s. This time delay is compensated numerically at the signal processing stage. The central frequency and relative bandwidth of the elements are 15 MHz and 60%, respectively. For data acquisition, the multi-channel ultrasonic pulse-echo flaw detector was used (tessonics.com). All channels record data 40 times per second (25 ms period). The main multi (52)-element transducer dimensions are shown in Fig. 3(a). Figure 3(b) shows the dimensions/ direction of the applied coating relative to the transducer elements. The gray region is the total coating with lines 1 and 2 showing the middle of the coating and highest point of the coating.

2.3 Sound Velocity and Density Determination Before progressing any further, there is a very important issue to discuss and that is the subject of sound velocity determination and heating effects during CS. It is well documented that sound velocity is dependent on the temperature of the material and with CS; the impacting particles will cause the substrate temperature to increase. Despite this, the current online monitoring method does

Fig. 3 Multi-array 52 element ultrasonic sensor. (a) Layout of transducer channels (distances in mm), (b) applied coating (gray area is the deposited material) and regions of interest (middle line—the coating axis, highest point—the coordinate of maximal coating thickness)


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from the top surface of the substrate. The time delay of this pulse relative to the pulse A is

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not account for temperature. However, this may not be as large a concern as it first may seem. Literature data about temperature coefficients of the sound velocity in polycrystalline metals and alloys are very rare. Besides that, the velocity and its temperature coefficient depend on the chemical composition and structure of the material. To determine the effect of a temperature change on the coating thickness measurement results, we estimated the temperature coefficient in a separate experiment. The velocity was measured in a standard pulse-echo setup as a function of temperature for the samples in a range of 20-80 C. The sample was deposited on the substrate using the same parameters of the deposition process. The measured velocity was found to be Cm = 4500 ± 60 m/s at 20 C, and the temperature coefficient was estimated to be in a range of nm 0.8… 1.4 m/(s Æ C). Thus, if the temperature of the samples during the deposition process does not exceed T* = 100 C, decrease of the velocity can be estimated as dCm = nm(T* - 20) 110 m/s. Neglecting temperature dependence of the velocity, we gave a relative error of the thickness measurement less than 2.5 %. It is obvious that the time delay being studied is actually between the top of the substrate and the added coating, which depends on sound velocity in the coating. For this reason, the sound velocity and density determination is considered to be of great importance. With this aim first, a portion of each sample was used to determine density. Using a YDK01 (Sartorius) density determination kit and the Archimedean principle, the density was determined to within an accuracy of 0.1%. The remaining portion of the samples was used to determine the longitudinal velocity of sound. The samples were ground and polished to give two parallel surfaces. The samples were then studied using an AM 1102 acoustical microscope (Tessonics) utilizing a 20 MHz flat transducer with a diameter of 3 mm equipped with flat delay line transducer V208 (Panamerics NDT Corp) having 20 MHz central frequency, 80% bandwidth, and 3 mm diameter of the piezoelectric element. Due to the porous nature of the material, it was undesirable to immerse the samples, and therefore Imagel R03-GEL1 (Tessonics) was used. The velocities were then determined by multiple trials (to prove accuracy) of the pulse echo overlap method. One can note for low pressure CS that since the particles are injected in the divergent part of the nozzle, they are not in the carrier gas for a long period of time and therefore will not attain a high heat. This effect will have to be studied in further detail but for now, it is believed that it should not have a large effect on the results.

3. Results and Discussion Examples of the data s(t,T) recorded by the element #6 (Fig. 2) are presented in Fig. 4 and 5. Figure 4 shows the data as a gray scale image (so called B-scan). Bright pixels correspond to high level of the signal, dark pixels correspond to negative values, zero level of a signal looks like

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Fig. 4 Grayscale representation of the s(t,T) function recorded by element # 6 (see Fig. 2): (a) sample with proper bonding (adhesion strength 40 Mpa); (b) sample with weak bonding (adhesion strength 15 Mpa)

gray background. The vertical axis is time of flight t (‘‘fast time’’) (propagation time of the ultrasonic wave), and the horizontal axis is the time of the process T (‘‘slow time’’). For clarity, two waveforms obtained at T = 4 s and T = 7 s are shown in a separate graph (Fig. 5). The results reveal that when the nozzle is approaching the area where the probe is attached, the temperature of the substrate and delay line is increasing. The sound velocity C0 (Eq 1) in the material of the delay line decreases with increasing temperature. This causes increases in the time delay tA, and gradual shifting of the pulse A as it is seen in Fig. 4 and 5. The time delay tB of pulse B relative to pulse A also changes with temperature, but for substrate materials such as steel and aluminum, the temperature coefficient of Cs is much smaller than that of the polymer delay line (Ref 12). For this reason, the variation of tB due to temperature change is neglected in this work.


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1.5

6

0.14 1

7

0.12 0.1

τ, μ s

s(t), a.u.

0.5

0

0.08

8 0.06

9

0.04

-0.5

10

0.02 -1 0 -1.5 0.8

-0.02 0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8


2

4

6

8

10

12

14

x, mm

0.65

Time delay t , µs

In a range of process time T = 5.5-7 s, the nozzle passes over the monitoring transducer. Due to the deposition of the coating material, we can see echo C in which time delay experiences rapid growth. Since we achieve a proper bonding, the substrate and the deposited layer, and the acoustic impedances of the aluminum substrate and deposited material (Al + 25%Al2O3) are very close. The amplitude of pulse B reflected at substrate/coating interface is small and the interface is not visible in the pictures after the deposition (Fig. 4a). The results of adhesion tests described in detail in monograph (Ref 7) show that the adhesion strength of Al + Al2O3 coating is about 40-60 MPa. Thus, a transparence of the interface looks like an indication of the coating bonding. If the proper bonding is not achieved the line of the pulse B shows the reflection Bm at the substrate/coating interface while the next line C shows the reflection from the top of the deposited layer (Fig. 4b). To estimate the thickness of the coating h(x,y,T), time delay s can be found by subtraction of the time delay of the B pulse measured before deposition from the delay of the pulse C relative to the pulse A: s = tC tB. After pulse B and C, several smaller pulses are visible (B1, B2,…, and C1) (Fig. 4). These pulses are produced by multiple reflections (reverberations) inside the substrate (or coating) and are neglected in the future analysis within this work. However, these data are believed to contain information about dissipation processes during passing ultrasonic waves. Detailed analysis of these results will be made in the following work. Figure 6 depicts calculated time delays of pulse C, which are proportional to the thickness of coating at a constant sound velocity (Eq 3). Determination of sound velocity in the deposited material was made with separate tests (Ref 13). Figure 6(b) shows important data, that of the actual coating build-up for each of the channels (shown in Fig. 3) considered. It shows the time delay t of the coating as a function of the distance the spray nozzle has traveled. In addition, each channel records thickness build-up at different distances since the

0

(a)

t, μ s

Fig. 5 Waveforms s(t) measured by element # 6 (see Fig. 2) at the time T = 4 s (dash line) and T = 7 s (solid line)

11

0.63

Channel 10 Channel 16

0.61

Channel 24

0.59

Channel 30 Channel 37

0.57

Channel 43

0.55 0.53 0.51 0.49 3.9

(b)

4.9

5.9

6.9

7.9

Time sprayed T, s

Fig. 6 Time delays s recorded: (a) by channels 6, 7, 8, 9, 10, 11 (see Fig. 2); and (b) by channels 10, 16, 24, 30, 37, 43 (see Fig. 3)

spray nozzle is traveling and therefore passes across each channel at a different time. This difference in fact allows us to estimate the nozzle speed during spraying simply by comparing the time at which build-up begins for channels in a line (by choosing a line parallel to spray direction). The velocity of ultrasound Cm in the deposited material is shown in Fig. 7 as a Cm/Cmo dependence on Al2O3 content for Al-Al2O3 coatings listed in Table 1. Cmo is the theoretical sound velocity of the Al-Al2O3 composite material defined with Hashin-Shtrikman equation (Ref 14). The results reveal the sound velocity in cold sprayed Al-Al2O3 composites is lower than in solid ones by 15-25%. This phenomenon seems to be the result of a lack of interparticle bonding by cold spraying. It is evident there is no correlation between sound velocity and coating density (Fig. 7). More detailed analysis of sound velocity measurement results for cold sprayed composites will be made in the ongoing work. The experimental sound velocity data obtained were used for present calculations. Figure 8 shows the calculated deposition rate a according to (Eq 5) for channels 6, 7, 8, 9, 10, 11 (Fig. 2). It can clearly be seen (Fig. 8) that the deposition rate in X and Y directions depend on the powder jet position which is defined by arrangement of the ultrasonic transducers in the X-Y plane. The deposition rate in the X direction is higher for the points located at the right side of a powder spot axis (see Fig. 1). It appears that a coating

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0.29

0.85

0.28 0.8 0.27 0.75

0.26

0.7

0.25 0.24

0.65

Porosity

Relative sound velocity Cm /Cm o

0.23 0.6

relative sound velocity

0.22

porosity

0.55

0.21 0.2

0.5 0

0.1

0.2

0.3

0.4

Alumina content

Fig. 7 Sound velocity and porosity measurement results for the coatings listed in Table 1

Fig. 8 Distribution of the deposition rate a(x,y) for channels 6, 7, 8, 9, 10, 11 (Fig. 2)

(a) 0.4 0.35 0.3 0.25

h, mm

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0.3

0.9

0.2 0.15 0.1 0.05 0 0

2

4

(b)

6

8

10

y, mm

Fig. 9 Coating cross section (optical image) (a) and thickness distribution (b). Diamonds—results of the h(y) measurement using an optical microscope; circles—results of calculation of the thickness h using delays of ultrasonic pulses s (the origin of the Y axis in (a) and (b) is different)

build-up changes an angle between the substrate surface and nozzle axis in the X-Z plane, which results in the increase of a. Therefore, it is possible to optimize the conditions of coating formation by controlling the nozzle axis angle in the X-Z plane. Variation of the deposition rate in the Y direction (Fig. 8) results in different coating thicknesses. To validate the measurement according to the proposed method, a cross section of the build-up has been made (Fig. 9a) and the profile h(y) has been measured

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using an optical microscope (Fig. 9b, diamonds). The cross section has been made outside of the transient area where the thickness is uniform along X axis. Calculation of the thickness h using delays of ultrasonic pulses s is shown in Fig. 9(b) (circles). The results reveal non-symmetry of the deposition rate a and the h(y) profile. The profile demonstrates a triangle-like shape and the deposition rate a has the same, which means that the deposition rate is higher in the center because of higher particle velocity and concentration. Figure 10 depicts the comparison of real


y = 0.9606x + 0.0227 2 R = 0.9928

Real thickness, mm

1.6

y = 0.9727x + 0.044 R2 = 0.9838

1.4 1.2

y = 1.0119x + 0.0124 R2 = 0.9975

Al+25%Al2O3+10%Zn

1 0.8 0.6 0.6

Al+30%Al2O3

Al+10%Al2O3 0.8

1

1.2

1.4

1.6

1.8

Acoustic thickness, mm

Fig. 10 Results of acoustic and real thickness measurements of Al based cold spray coatings. Circles (approximation y = 0.9606x + 0.0227)—Al + 10%Al2O3 coating; diamonds (approximation y = 0.9727x + 0.044)—Al + 25%Al2O3 + 10%Zn coating; triangles (approximation y = 1.0119x + 0.0124)—Al + 30%Al2O3 coating

thickness measurement data and acoustic thickness calculated using delays of ultrasonic pulses recorded by the probe shown in Fig. 3. The acoustic thickness values might not be the exact same but the linear relationship for various data points show that this is viable. Another important point to note is that many of our data sets have y intercepts in the range of 0.01-0.004 mm, which you would expect since a 0 real thickness means no coating and obviously should lead to no acoustic thickness. This will obviously mean that some type of correction factor will have to be introduced to set the intercept at 0 to increase the accuracy of the method. In addition, it may not even be necessary if a database of points is made since it will be clear that a given acoustic reading corresponds to a set thickness. That being said, a look at the online measurements shows some very promising data. The linear fit is fairly good. This means the online data is giving the closest fit to what is expected between acoustic and real thickness.

4. Conclusion The acoustic analysis showed great promise. Not only can the final value of thickness be estimated, but it is also possible to see the dynamics of how the build-up takes place in real time. The results demonstrated that the


References 1. C.F. Rocheville, US Patent, 1963, 3100724 2. R.C. McCune, W.T. Donlon, O.O. Popoola, and E.L. Cartwright, Characterization of Copper Layers Produced by Cold GasDynamic Spraying, J. Thermal Spray Technol., 2000, 9(1), p 73-81 3. A.P. Alkhimov, V.F. Kosarev, and A.N. Papyrin, A Method of Cold Gas-Dynamic Deposition, Sov. Phys. Doklady, 1990, 35(12), p 1047-1049 (in Russian) 4. R.C. McCune, A.N. Papyrin, J.N. All, W.L. Riggs, and P.H. Zajchowski, An Exploration of the Cold Gas Dynamic Spray Method for Several Materials Systems, Proceedings of the 8th National Thermal Spray Conference, C.C.Berndt, Ed., Houston, TX, USA, ASM International, 1995, p 1-5 5. V.K. Champagne, Ed., The Cold Spray Materials Deposition Process: Fundamentals and Applications, Woodhead Publishing Limited, Cambridge, 2007, p 230-312 6. A. Papyrin, V. Kosarev, S. Klinkov, and A. Alkhimov, Cold Spray Technology, Elsevier, Amsterdam, 2006, p 125-210 7. R.Gr. Maev and V. Leshchynsky, Introduction to Low Pressure Gas Dynamic Spray, Wiley, Weinheim, 2007, p 75-110 8. H. Assadi, F. Gartner, T. Stoltenhoff, and H. Kreye, Bonding Mechanism in Cold Gas Spraying, Acta Mater., 2003, 51, p 43794394 9. J. Karthikeyan, C.M. Kay, J. Lindemann, R.S. Lima, C.C. Berndt, Thermal Spray 2001: New Surfaces for a New Millenium, C.C. Berndt, Ed., ASM International, OH, 2001, p 383-387 10. R.Gr. Maev and V. Leshchynsky, Air Gas Dynamic Spraying of Powder Mixtures: Theory and Application, J. Therm. Spray Technol., 2006, 15(2), p 198-205 11. M. Lubrick, R.Gr. Maev, V. Leshchynsky, Development of Cold Spray Composite Coatings Non Destructive Characterization, MS&T 2008, Pittsburgh, PA, USA (CD proceeding), 2008, p 78-84 12. R.Gr. Maev, Acoustic Microscopy, Wiley-VCH, Weinheim, 2008, p 170-210 13. M. Lubrick, R.Gr. Maev, and V. Leshchynsky, Youngs Modulus of Metal-Matrix Composites Made by Low Pressure Gas Dynamic Spray, J. Mater. Sci., 2008, 43, p 4953-4961 14. H.X. Peng, Z. Fa, and J.R.G. Evans, Bi-Continuous Metal Matrix Composites, Mater. Sci. Eng., 2001, A303, p 37-45

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build-up process is universal across the spray, with slower build-up at the outer extremities of the coating. Estimates of nozzle speed and spray diameter matched fairly well with the actual values and should be easy to improve by aligning the spray completely parallel to the line of channels considered. Also, the cross-sectional thicknesses show very accurate profiles of the actual coating structure. Most importantly, it was shown that comparing real and acoustic thickness led to a reliably linear fit for all data points. It clearly is shown that thickness estimates from acoustic data are a viable method.

1.8